Chart isn't cleared when no data is received

[pgiraud]

Steps to reproduce:

• display an instance overview dashboard (ie. http://0.0.0.0:8888/server/0/overview/): data is displayed in the general overview charts.
• use the date picker to choose a date where there's no data collected (for example in the future).

The "No data" message is displayed but the old data is still there. Even worse, the chart loading spining image keeps being displayed if you choose dates for which there is no data several times in a row and then go back to a valid date.