Drawing a graph with grid lines

The drawing of grid lines is performed by the axis painters of the graph. ...
In order to turn on a grid, one has to specify in the argument `gridattrs` a
list of attributes defining the way the grid should look like. In order to
enable a grid at the position of the axis ticks and subticks, it is enough to
set `gridattrs` to an empty list. In the example, this is done for the grid
corresponding to the x-axis. If one wants to control the style of the grid
lines or even turn them off selectively for the main or subticks, one can pass
`attr.changelist` instances in the list. The first item of the argument of this
class is then used for the main ticks, the second for the first subticks, and
so on. In the example, it is shown how to change the linestyle of the grid at
the main tick positions and to turn off the grid for the subticks by setting
the correpsonding value in the `attr.changelist` instance to `None`.

! The example also shows a neat trick how to create an axis with axis labels at
fractions of pi. This can be achieved by passing a `divisor` argument to the
axis itself, which means that for the axis handling, all values are divided by
the corresponding value. Then, we automatically put ticks at the position of
rational number by means of `axis.texter.rational` and add the missing factor
pi at the end of the TeX output.
