The automatic tick generator AxisItem.tickSpacing() has a few quirks:

• For small plots, it often generates only a single labeled tick. That makes the tick spacing indecipherable.
• For particularly unlucky configurations, it generates no labeled ticks. This usually collapses the space reserved for the axis, while it is presently in use for zoom or pan.
• The code now only supports multiples of 1 and 2 per decade, creating a five-fold jump in tick density when crossing from 10 to 2. Including multiples of 5 reduces this jump, but occasionally results in unevenly spaced tick labels like [2, 4, 5, 6, 8] where the not fully overlapping sequences of major and minor ticks both get labeled.
• For long axes, very few major ticks are generated. A different part of the code often considers these too sparse and activates emergency text labels for each tick. Down to the third-rank extra ticks, that would not usually be considered for labelling.

These issues intensify when working with larger font sizes, particularly because labels current cannot extend beyond the height/width of the viewbox, and (large) labels near the edges are culled.

Here is a demo to play with some of the issues:

import numpy as np
from PyQt5.QtGui import( QFont )
import pyqtgraph.pyqtgraph as pg
from pyqtgraph.pyqtgraph.Qt import QtCore

app = pg.mkQApp("Axes issues demo")

x = np.arange(30) + 123000
y = 0.5 * np.random.normal(size=30)

win = pg.GraphicsLayoutWidget(show=True, title="axes demo")
win.resize(1000,800)

p1 = win.addPlot(title="Only one tick label", x=x, y=y, row=0, col=0)
p1.setYRange(-1, 1, padding = 0.)

p2 = win.addPlot(title="sometimes ticks are dense...", x=x, y=y, row=0, col=1, rowspan=2 )
p2.setYRange(-1.75, 1.75, padding = 0.)

p3 = win.addPlot(title="... sometimes very sparse", y=y, row=0, col=2, rowspan=3 )
p3.setYRange(-2.10, 2.10, padding = 0.)
axis = p3.getAxis('bottom')

p4 = win.addPlot(title="It's worse with large fonts", x=x, y=y, row=1, col=0)
font = QFont()
font.setPointSize(25)
axis = p4.getAxis('left')
axis.setTickFont(font)
p4.setYRange(-0.5, 9.5, padding = 0.)

p5 = win.addPlot(title="wonky with long x-axis labels (pan to see)", x=x, y=y, row=2, col=0, colspan=2)
p5.getAxis('bottom').setTickFont(font)
p5.setXRange(123010, 123019.5, padding = 0.)

p6 = win.addPlot(title="Emergency labels on third rank ticks", y=y, row=3, col=0, colspan=3)
axis = p6.getAxis('bottom')
if hasattr(axis,'setTickDensity'): axis.setTickDensity(5.0)
axis = p6.getAxis('left')
if hasattr(axis,'setTickDensity'): axis.setTickDensity(4.0)
p6.setYRange(-1, 1, padding = 0.)

if __name__ == '__main__':
    pg.exec()

Changes

This PR replaces the generation code:

• It starts by creating a major spacing that ensures(*) that two ticks are displayed.
• If the best major spacing is 1, then the minor spacing will be 0.5.
• If the best major spacing is 2 or 5, then the minor spacing will be 1. This usually avoids having non-aligned spacings of 2 and 5 in the same plot.
• Sub-minor (extra) ticks are generated at 10% of the major spacing.
• Undersized spacings (of less than 2 pixels on screen) are enlarged, extra ticks are only shown if visible.
• An added method setTickDensity() makes it possible to adjust the auto-generated ticks somewhat without full manual specification.

(*): This still makes implicit assumptions on text lable sizes, but referencing the actual sizes would be rather complexity.

Results

Current master branch:

Revised axis ticks: