Overlapping data series are not rendered nicely

When selecting just two dataseries in the charts, everything looks rather good:


However, when adding more data series, the colors are mixed awkwardly, making it hard to distinguish them:


This is due to the use of transparency in the fill colors. Perhaps, we should look at [Chart.js’s area charts](http://www.chartjs.org/samples/latest/charts/area/line-datasets.html), which seem to handle color transparency much more nicely:


In the worst case, we should consider removing the fill colors entirely, as legibility of the charts should be top priority.