To see the true midpoint, I always create the chart as a "combined chart," rather than charting the spread directly.
Go into the chart parameters, then at the very top, change it from "Single Contract" to "Combined Contract" and add your legs individually. I also like to manually set the scale, as auto scale tends to be horrible on these. One thing I have yet to figure out is how to keep the scale from getting f'd up when adding the underling graphed against a secondary axis.