Posting code is more useful than screen shots, though sometimes both code and screenshots are helpful.
That said, from what I can see:
- you have one plot
- you have two series (lines drawn based on x,y data) on that plot
- you have two axis on the plot. Both are on the left.
-
- the first series (series[0]) is plotted against axis[0]
-
- the second series (series[1]) is plotted against axis[1]
From your description, what you want is probably
- one plot
- three series (measurement, USL, LSL)
- one axis, with all three series plotted against the same axis (axis[0])
You have explicitly instructed Perspective to plot series[1] on axis[1] by selecting USL as the yAxis.
Change that to yAxis: default (or whatever you named axis[0]) and see if that is what you were expecting.