Fix bug in xticks for Table.plot()
Fix bug in xticks for Table.plot()

Before, calling

    Table([np.arange(0, 10, .1), np.arange(0, 1, .01)], ['x', 'y']).plot('x')

Would show xticks of 0, 0.1, 0.2, etc. instead of 0, 2, 4, ...
This was beacuse we assumed that the horizontal axis values as passed in
the `ticks` variable were integers. Instead, we should just ask the axis
for the ticks it used to plot the data.

To test:

```python

a = Table([np.arange(0, 10, .1), np.arange(0, 1, .01)], ['x', 'y'])
a.plot('x')

b = Table([np.arange(0, 20, .1), np.arange(0, 2, .01)], ['x', 'y'])
b.plot('x')

dsten/datascience,data-8/datascience