Powder plotting: save limits, tickmark & diff location in the histogram object

[briantoby]

From ORNL staff member: "I find I can drag the residual and adjust zoom to get a perfect view of the data and the fit... But, then if I flip to the other bank, I get..." [limits don't change, ticks move, but diff xfers between histograms].

What would be better would be that the first time a histogram is displayed, these settings are initialized (also when one switches to sqrt plot), but thereafter are saved. Placing these values into the histogram dict somewhere should do this.

[briantoby]

Limits may be a bit tricky as they are stored by MPL. How does one find that they have been changed or save them before switching histograms?