Allow option to change zoom level and placement about cartogram-chart

[vasturiano]

@jalperin thanks for reaching out. You can control the map translation and zoom using the projection itself (set by the projection() method).

This is the code for the default projection:
https://github.com/vasturiano/cartogram-chart/blob/master/src/cartogram.js#L18

As documented, it defaults to:

geoMercator (centered on prime meridian, slightly tilted towards the northern hemisphere)

If you wish to change the zoom/scale for example, you can just do:

myChart.projection().scale(6)

You can also use the above method to use a different projection other than geoMercator.

[jalperin]

small correction to the above, I think. This is what it needs to look like:

.projection(d3.geoEquirectangular().scale(200))

[vasturiano]

@jalperin that's only if you need to change the projection. If you just want to change the scale then the previous code should suffice.

[jalperin]

I was getting the following error:

(index):173 Uncaught TypeError: Cartogram(...).topoJson(...).projection(...).scale(...).topoObjectName is not a function
    at update ((index):173)
    at <anonymous>:1:1

[vasturiano]

That's because you can't chain chart methods after calling the projection getter (the return object is then a projection instead of the chart object). Just place the chart methods before the .projection() and it should be good.