Rendering a graph view with several options selected for a given
date groupby can take too much time and block the page.
Since grouping on the same date/datetime field several times
barely makes sense in a graph view, we choose to use only the finer
option, e.g. 'week' is prefered to 'quarter'.
Task ID: 2198846
X-original-commit: 7e36bee14e18ca17e15de1bacb7ff26010d0b372