stack line plot marker tooltips for markers that are very close to each other

[danicahelb]

some line plot markers differ by very little on either the x- and/or y- axis value to the point where they look equal but aren't. the only way to access the tooltip for the one you want is to toggle things off in the legend.

when multiple markers have the exact same x- and y- axis values we stack the tooltips VEuPathDB/EdaNewIssues#547

it would be good to also stack tooltips for markers that are very close to each other

this is not currently a priority for implementation