Ensured that Y-axis settings are persistent when closing/re-opening PICSA Rainfall dialog

[Vitalis95]

Fixes #7607
@rdstern @N-thony @lloyddewit this PR fixes the issue when you set Y-axis at any point then it doesn't unset itself when dialogue is used repeatedly

[rdstern]

@Vitalis95 this is much better and the control now seems to behave itself when the y-axis is numeric. But it still goes wrong when using dates. I will send you the data for the plot below through skype.

It would be excellent if you can fix this too, at the same time?

To be clear what I did. I first did plots where y is the sum_rain. I get the mean line and also set the y-axis to start at zero. All seems ok now.

Now I change and plot the start_rain. Now the y-axis is a date, and the default is for the y-axis to start at day 1. I untick this.
The graph works fine.
Then I return to the dialogue and the PICSA plot sub-dialogue. Then I go to the Lines tab. I don't change anything, but do the plot again. Now the graphs start on Jan 1. When I return the the PICSA sub-dialogue I find the y-axis control is ticked and at the default of 1. I hope you can prevent that?

[Vitalis95]

@rdstern , can you have a look at it now

[rdstern]

@Vitalis95 excellent. Looks fine now. Many thanks. Hope @lloyddewit likes it so it can be merged.