Axis `nticks` description is wrong

[yankev]

http://codepen.io/plotly/pen/92268ebeb73d75262fbebfdcd8e64450?editors=1010

In the example there are 18 data points, and nticks (for the x-axis) has to be set to 19 before there is one tick per bar.
Other values for tick < 19 result in other unexpected behaviour.

[etpinard]

Same issue as #614

nbins and nticks (and ncontours) for that matter use the same algorithm.

This is the intended behaviour, not a bug.

We should change the description here though.

[cldougl]

and here then as well

[etpinard]

fixed in #662