To change the legend labels, I create a formula with a descriptive formula name and plot it on the graph.
For example, let's say that I would want to plot the count of a field named {tableName.myId} and I would want the label "Count of Ids" in my legend.
I would first create a formula named "Count of Ids" and in it I would write Count({tableName.myId}). Then, I would plot that formula in the chart, and the legend label would be "@Count of Ids". It's impossible to get rid of the "@", though.
Hope that helps!