Show downloads estimation for incomplete months & years

[graphieros]

UX issue

• users experience charts with downward trends for incomplete years or months, in the chart modal

Solution

• inject a dashed line connecting n-1 to an estimation for the complete granularity, only if the period is incomplete (basically almost all the time)
• impact: since the scale is computed from the dataset, an estimation that would exceed the max scale would overflow the top of the chart. The maxScale should be set from max(estimation, maxDatapoint) but also normalized to a nice scale.

[graphieros]

closed by #1177