plot_pixels with no pixels has confusing error

[delucchi-cmu]

Bug report

If you have a catalog with no remaining pixels (e.g. a cone search that lies outside the survey footprint), and you attempt to plot the pixels, you get an error way on down inside the libraries.

We could make a quick check that there are some pixels before attempting plotting and throw a friendlier error.

File ~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:241, in HealpixDataset.plot_pixels(self, **kwargs)
    [235](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:235) def plot_pixels(self, **kwargs):
    [236](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:236)     """Create a visual map of the pixel density of the catalog.
    [237](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:237) 
    [238](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:238)     Args:
    [239](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:239)         kwargs: Additional args to pass to `hats.inspection.visualize_catalog.plot_healpix_map`
    [240](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:240)     """
--> [241](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/catalog/healpix_dataset/healpix_dataset.py:241)     return plot_pixels(self, **kwargs)

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:101, in plot_pixels(catalog, plot_title, **kwargs)
     [99](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:99) pixels = catalog.get_healpix_pixels()
    [100](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:100) default_title = f"Catalog pixel density map - {catalog.catalog_name}"
--> [101](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:101) return plot_pixel_list(
    [102](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:102)     pixels=pixels,
    [103](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:103)     plot_title=default_title if plot_title is None else plot_title,
    [104](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:104)     **kwargs,
    [105](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:105) )

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:124, in plot_pixel_list(pixels, plot_title, projection, color_by_order, **kwargs)
    [122](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:122) ipix = np.array([p.pixel for p in pixels])
    [123](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:123) order_map = orders.copy()
--> [124](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:124) fig, ax = plot_healpix_map(
    [125](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:125)     order_map, projection=projection, title=plot_title, ipix=ipix, depth=orders, cbar=False, **kwargs
    [126](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:126) )
    [127](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:127) col = ax.collections[0]
    [128](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:128) if color_by_order:

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:529, in plot_healpix_map(healpix_map, projection, title, cmap, norm, ipix, depth, cbar, fov, center, wcs, frame_class, ax, fig, **kwargs)
    [516](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:516)     depth = np.full(len(healpix_map), fill_value=order)
    [518](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:518) fig, ax, wcs = initialize_wcs_axes(
    [519](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:519)     projection=projection,
    [520](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:520)     fov=fov,
   (...)
    [526](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:526)     figsize=(10, 5),
    [527](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:527) )
--> [529](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:529) _plot_healpix_value_map(ipix, depth, healpix_map, ax, wcs, cmap=cmap, norm=norm, cbar=cbar, **kwargs)
    [530](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:530) plt.grid()
    [531](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:531) plt.ylabel("Dec")

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:629, in _plot_healpix_value_map(ipix, depth, values, ax, wcs, cmap, norm, cbar, **kwargs)
    [626](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:626)     depth_ipix_d[d] = (ipix[mask], values[mask])
    [628](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:628) # cull backfacing and out of fov cells
--> [629](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:629) fov_culled_d = cull_to_fov(depth_ipix_d, wcs)
    [630](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:630) culled_d = cull_from_pixel_map(fov_culled_d, wcs)
    [632](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:632) # Generate Paths for each pixel and add to ax

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:272, in cull_to_fov(depth_ipix_d, wcs)
    [254](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:254) def cull_to_fov(depth_ipix_d: dict[int, tuple[np.ndarray, np.ndarray]], wcs):
    [255](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:255)     """Culls a mapping of ipix to values to pixels that are inside the plot window defined by a WCS
    [256](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:256) 
    [257](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:257)     Modified from mocpy.moc.plot.utils.build_plotting_moc
   (...)
    [269](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:269)         and any pixels too small merged with their map values subsampled.
    [270](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:270)     """
--> [272](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:272)     depth_ipix_d = _merge_too_small_pixels(depth_ipix_d, wcs)
    [274](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:274)     moc_viewport = get_fov_moc_from_wcs(wcs)
    [276](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:276)     if moc_viewport is None:

File ~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:305, in _merge_too_small_pixels(depth_ipix_d, wcs)
    [302](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:302) depth_res = int(np.floor(np.log2(np.sqrt(np.pi / 3) / cdelt)))
    [303](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:303) depth_res = max(depth_res, 0)
--> [305](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:305) max_depth = max(depth_ipix_d.keys())
    [307](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:307) # Combine healpix pixels smaller than 1px in the plot
    [308](https://file+.vscode-resource.vscode-cdn.net/home/delucchi/git/smoke/lsdb/~/git/smoke/hats/src/hats/inspection/visualize_catalog.py:308) if max_depth > depth_res:

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• I have described the situation in which the bug arose, including what code was executed, information about my environment, and any applicable data others will need to reproduce the problem.
• I have included available evidence of the unexpected behavior (including error messages, screenshots, and/or plots) as well as a description of what I expected instead.
• If I have a solution in mind, I have provided an explanation and/or pseudocode and/or task list.