Displaying minutes and seconds on a line graph

[jamesintulsa]

In my data set for a call center, I have minutes and seconds that I need to display on a line graph.  In my data it shows 00:30:15 for example.  But when I try to graph that it only shows a count (1).  I go to edit and try to change to duration, but it just shows 00:00:00.  It is showing in the setup the Metric is CT.