Generate dynamic histogram plot on timeline map

[ect123]

the existing graph doesn't line up exactly with the slider.

Dates aren't calibrated to each year – maybe we add 0 values in the data sheet for years without composers?

[epoudrier]

Merge column pre-1900, delete "unknown" column. Increase size of text.

[craigsapp]

Craig will do via vega-lite (https://vega.github.io/vega-lite).