Jagged curves on graph when frequency is high

[KatieWoe]

Test device:
Dell Laptop
Operating System:
Win 10
Browser:
Chrome
Problem description:
For phetsims/qa#238
The graph of a wave down the center can be rather jabbed, particularly when the frequency is high as with the blue colors of light. @arouinfar asked for an issue to see if it could be cleaned up, but it may not have enough points and so it may not be possible.
Steps to reproduce:

1. Go to the Slits screen (waves stay larger for longer, so its easier to see)
2. Go to the light scene and set the frequency as either a blue or a violet
3. Start the wave and turn on the graph

Screenshots:

Troubleshooting information (do not edit):

Name: ‪Wave Interference‬
URL: https://phet-dev.colorado.edu/html/wave-interference/1.0.0-dev.51/phet/wave-interference_en_phet.html
Version: 1.0.0-dev.51 2018-12-12 03:47:24 UTC
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Pixel Ratio: 2.5/1
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Max viewport: 16384x16384
OES_texture_float: true
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[samreid]

If I recall correctly, we experimented with different smoothing strategies and found this was preferable. There is a very small number of cells in the lattice that is used to generate the curve, hence that is why it is jagged--I'm not sure what else we can do with this. @arouinfar any thoughts?

UPDATE: I guess we could apply postprocessing smoothing on the curve, but we would need to take care to make sure the water side view always exactly matches the graph curve. But I think we tried postprocessing smoothing and it flattened out peaks too much.

[arouinfar]

I asked @KatieWoe to open this issue so I could hear @samreid's thoughts on the issue. Thanks for jogging my memory @samreid. Light suffers more than the other scenes because the wavefronts are more closely spaced, but I agree that there isn't anything else we can do here.