flint: Fallback to anyway display something in the curve plot
Closes #3539 (closed)
This PR fixes 2 bugs which was breaking the expected behavior of the curve plot selection.
Now a counter will anyway be selected by default if finally nothing match.
This ensure that something will be displayed in the plot, including during sequence of scans like:
loopscan(10, 0.1, diode1)
loopscan(10, 0.1, diode2)
loopscan(10, 0.1, diode1)
Previously it was needed to tune properly plotselect
vefore the scan.