Replace Plotly+sparkline by Dygraph
Closes #7 (closed)
Since this is not the first issue with sparklines, in order to remove the discrepancies between the different charting libraries, this MR replaces Plotly and sparkline by dygraph.
In addition, the dygraph library is also lighter.
The download as PNG feature was reimplemented using html-to-image and filesaver.
The period alignment was reimplemented client-side and now distinguishes real NA values from holes in series due to period alignment (represented as null
values, cf https://dygraphs.com/tests/independent-series.html). As a result, the API responses are lighter.