extract out tickvalue definitions for line chart so it is configurable

[golgistudio]

Currently, the line chart assumes that the x axis is dates and sets the tick values with that assumption.

Refactor so that is configurable

https://github.com/codeforsanjose/economic-indicators-dashboard/blob/master/src/charts/lineChart.js

[golgistudio]

Extracted out for the linechart

x-tick-interval can be set to either 'months' or 'quarters'