The error
ax.XAxis.TickValues=1:10:length(NewP{:,"Date"})
Error using matlab.graphics.axis.decorator.DatetimeRuler/validateTicks
Value must be a vector of increasing datetime values.
comes from the use of the value '10' -- the XAxis is a DateTimeRuler object as it says in the error message because you used a datetime as ordinate variable. Presuming the 10 is intended as 10 days you need
ax.XAxis.TickValues=1:days(10):length(NewP{:,"Date"})
instead.
To eliminate the line between the two data sets, the simplest thing to do would be to plot the two sets independently instead of joining them:
figure
plot(P1{:,"Date"},P1{:,"MeltCushion"}
hold on
plot(P2{:,"Date"},P2{:,"MeltCushion"}
where I used P1 and P2 as placeholder variables for your first two variables as used in the first example plot. The only difference is to use the date variable instead of just the time variable as you did there and not joining the two data sets together. You'll also get a color difference automagically, you can set the line color as desired if want them the same.
The problem you'll still have will be that as in your present second graph, the data in each day itself is going to be almost indistinguishable as just a smudge because the time difference between the two dates is so large compared to the time within the day so the linear axis scale isn't appropriate for the detail.
An alternate would be to use an arbitrary day difference between the two and then adjust the tick labels to reflect actual dates if those are important.
