[plot] Readability issue with axes

[vallsv]

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2 feedback from ESRF scientifics on Flint.

The scientific notation is not used in axes which make the result difficult to read.

• Values like 300000 are displayed
• Values like 0.0000000000001 are displayed
• Worst, for small values like 0.0000000000001, as there is not enough spaces, the value is truncated and all the labels of the axis can contains 0.0000000 (which is pointless)

I could be nice to use at least scientific notation:

• for numbers > 1000
• for numbers < 0.01
• We could try to use back the offset concept
  • The data tooltip have to provide the right value anyway
  • Maybe display it for every labels 3x10^5 + 0.1
  • Maybe explicitly display that there is an offset OFFSET + 0.1

There is no good solution i guess. But we could improve the result.

And this could be improved in silx.