Represents one axis.
Object setings:
min: Minimum value for the bottom or left end of the axis
max: Max value.
labels: List of labels to show along the axis.
label_positions: List of positions to show the labels at. Uses the scale
set by min & max, so if you set min = 0 and max = 10, then
label positions [0, 5, 10] would be at the bottom,
middle, and top of the axis, respectively.
grid_spacing: Amount of space between gridlines (in min/max scale).
A value of 0 disables gridlines.
label_gridlines: If True, draw a line extending from each label
on the axis all the way across the chart.