Some comments about the pie chart:
The category name is lost as soon as you click on Ok. When you open the popup again, the names are just "Category 1", "Category 2", so there isn't much sense in spend much time writing something there.
The pie chart always takes the whole space. If you select an area, it won't make the chart smaller, but just render that specific part of the graph, which in my opinion, doesn't make much sense.
It would be great if the existing categories list support multi select, so you can remove more than one at the time. Or if there was a "remove all" or "clear" button. Apparently there is no keyboard shortcut to remove a category, so you have to use your mouse to click on an entry and then on the Remove button repeatedly.
It would also be great if clicking or double clicking on an existing category would reload the field above, so you can easily edit it, instead of having to delete and readd it.
Add always adds the category at the end. It would be great if it was added before the selected existing category, if any. But this requires that clicking on a selected category un-selects it, so you can still add a category at the end.
Too bad it's gone.