Well, not instantly & automatically without doing something.
If it is a "one off" exercise and your data is in a table (list), then one way would be to put a formula in another column. Say you have some contest identifier in column A and the results in column B. In another column a formula of the form =IF(column A in the row above = column A in this row, column B value, column B & " - winner")
Run this formula down your whole list. Then copy the results over the original column B data - using paste special values. Or alternatively change your chart to plot this new data instead of the original column B data.
You can do some looping in VBA to achieve a similar result programmatically.