There may be a way to do both together if you only want to display the top 10 altogether in the report. If not you will have to do a sub report for the top 10 chart.
Click on the Group SOrt Expert.
In the For this Group select Top N
based on: Count of...
Where N is: if you put in 10 it will only show the top 10 otherwise you can put in a number higher than the total records to display them all and sort them all.
Include others with name: If you choose to display top 10 you can check this and it will clump all the non top 10 into a last grouping to still show in the report (it will also appear in your bar chart under the name you give it here).
For the Chart,
if you use the Top 10 just insert the chart and use the wizard to it will only display the top 10 (and others if you checked that box indicated above).
If you do not use the top 10 you will need to create a sub report that mimics this report exactly but does use the top10. Suppress everything in that report except the chart.