The integer keyword in MaxNlocator should do what you want. Have you tried it?

import matplotlib.pyplot as plt
import matplotlib.ticker as ticker

sci_nums = [10**9, 8*10**9]
fig, ax = plt.subplots()
ax.scatter(range(len(sci_nums)), sci_nums)
ax.yaxis.set_major_locator(ticker.MaxNLocator(4, integer = True))

I tried it, for my part it does not seem to work in the example you provided (Matplotlib version: 3.0.0rc1.post186+g5e01393)

The issue is actually in ScalarFormatter and its interaction with the extra ticks made by MaxNLocator. MaxNLocator(4) actually makes six ticks that are used for the orderOfMagnitude calculation, and the order of magnitiude of that larger span changes when you use 4 ticks instead of 6.

I don't actually think there is any reason to not just set the order of magnitude by visible ticks. Fix on its way...

Cross ref #11004

I am a little concerned by the handling of the integer keyword.

   *integer*
     If True, ticks will take only integer values, provided
     at least `min_n_ticks` integers are found within the
     view limits.

How can we be sure that only integer ticks are displayed if the formatter method axis.major.formatter does not know about the integer keyword provided to the tick locator, in that case maxNLocator?

@dabana the ticks being chosen by the locator are integers in both cases. It’s the formatting of those integers that needs fixing.

I guess I am confused with what the integer keyword is supposed to do. I thought all the tick labels would end-up greater than 1 without decimals (like 1, 2, 3 ... or 3, 6, 9, etc...).

By reading the code, I understand than only the tick labels greater than 1 will be integer.

        if self._integer:
            # For steps > 1, keep only integer values.
            igood = (steps < 1) | (np.abs(steps - np.round(steps)) < 0.001)
            steps = steps[igood]

If this is the correct behavior it might be worth rewording the doc_string to make this clear. For exemple:

integer
If True, ticks greater than 1 will take only integer values, provided
at least min_n_ticks integers are found within the
view limits.

Integer on the locator will simply make sure the tick locations are integers. You are conflating the scientific notation of the formatter with the job of the locator.

Thank you for fixing this @jklymak !!