This is not a very common Issue so just by going that screenshot of legend wouldn't help probably.
You can add more details, if the screenshot of the report would have been shared, may be I would have been in a better position to suggest solutions. Nevertheless, try below
1) If this is due to space issue, try extending your report and the chart length slightly by dragging the chart and then the report along length.
2) Change the legend positions and see is issue persists (Changing from bottom to say, Right Corner etc)
3) Try to give custom label names (not the default ones) and giving small names for testing, such as Label1, Label2, Label3 etc and see if issue still persists.
4) Change the values in parameters (if any) for the report so that value in report changes and if that affects reports. e.g. Viewing report once for Jan 2013 and then viewing it for 2013 whole year, or something similar.