What does a string like 1e-6+6.0459e-1 mean in the context of a vertical axis label in a matplotlib plot?

Question

I have a Python code that gives me plot like this one (made by matplotlib):

the mean value is about 0.6065

Then I change a parameter and, starting from a certain value, matplotlib automatically add a string like 1e-6+6.0459e-1 to the vertical axis:

What does it mean? Maybe that there is a "reference" y-value at +6.0459e-1 and there is an oscillation peak to peak of about 5e-6?