rosieluain
It is not obvious, so thanks for asking! The plotting widget is based on QCustomPlot and its C++ API is documented here. However, the full QCustomPlot API is not exposed in python -- we've just been adding things as we need them. So, if there is something you can see QCustomPlot is able to do but there does not seem to be a way to do it via python, just let us know and we can add in support.
A small example showing how to set the tick labels is below:
from iolite.ui import IolitePlotPyInterface as Plot
import numpy as np
x = np.arange(0, 10, dtype=np.float)
y = x**2
labels = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J']
p = Plot()
g = p.addGraph()
g.setData(x,y)
p.bottom().setTicks(x, labels)
p.rescaleAxes()
p.show()