Add histograms for population density

[kavigupta]

• seems like the domestic histograms aren't being generated for american states / urban centers
• add drawing of histograms
• add cumulative histograms
• add population scaling to histograms
• add toggle button
• make toggle button reliant on a setting
• per square mile
• fix the rounding
• add colors
• add hoverable values
• add tests for how the histograms display
• add tests for hoverable values (might be hard)
• add test for png download
• add tests for nans
• add segments for kinds of place (maybe not doing this)
• fix rendering of toggle button
• fix font size
• bar chart toggle
• fix bar chart spacing
• why does it say "name"
• legend
• gracefully handle nans
• no negative yaxes
• fix y axis
• make elements go away for screenshot
• wrong order???
• chart download