Matrix input to X vs Y plot sometimes shows extra traces

I put a set of arrays into an x vs y plot with matrix input enabled. One set is frequency (X axis) in GHz (range .01 to 18), and the other is VSWR (range 1.0 to ~1.2). The x and y axis arrays are the same size & shape (3 X 52). This set causes the x vs y plot to show extra traces, some looking like a scope retrace. The other data set is similar, power on the X axis (range -36 to +20) and % error on the Y axis (range ~ +/- 3%) with just the numbers, no "%". This data sets displays normally with no extra traces.  Does anyone have any clue what causes the extra traces to display and how to fix it?