n2o: enhancement of chart form and charts

[Olivier Maury]

After talk with Cécile,

at /n2o/extraction/charts, enhancements to the interface could be done:

- a dropdown list for choosing the modalities should be added,
- the dropdown list for chart type is not used, only one chart type is needed,
- the buttons are never used, as the chart is automatically changed,
- the chart background must use a more neutral colour, ex grey #f5f5f5.

After demo, new remarks:
- the legend is not ordered well and must be enhanced:
  temperature and precipitation are mixed with flux series,
- as name of the series, using "Flux SERIENAME" should be better.