(Three years late...) but I believe the answer to your second question is that SSRS essentially treats data from your datasets as unsorted; I'm not sure if it ignores any ORDER BY in the sql, or if it just assumes the data is unsorted.
To sort your groups in a particular order, you need to specify it in the report:
- Select the chart,
- In the Chart Data popup window (where you specify the Category Groups), right-click your Group and click Category Group Properties,
- Click on the Sorting option to see a control to set the Sort order
For the report I just created, the default sort order on the category was alphabetic on the category group which was basically a string code. But sometimes it can be useful to sort by some other characteristic of the data; for example, my report is of Average and Maximum processing times for messages identified by some code (the category). By setting the sort order of the group to be on [MaxElapsedMs], Z->A it draws my attention to the worst-performing message-types.
This sort of presentation won't be useful for every report but it can be an excellent tool to guide readers to have a better understanding of the data; though on other occasions you might prefer a report to have the same ordering every time it runs, in which case sorting on the category label itself may be best... and I guess there are circumstances where changing the sort order could harm understanding, such as if the categories implied some sort of ordering (such as date values?)
