[feature request] render xAxis like yAxis style Namely from discrete way to continuous way

[liuxuan30]

I am not sure if you can get notification in ios-charts if I @ you. So I create a post here.

Reference:ChartsOrg/Charts#176

Current xAxis limitation is it is using index to draw.

For example, when the xAxis represents date & time, I have data '10.0' at 07:05am, and have '20.0' at 07:05pm. If we use index technique, to reflect the xAxis span from 00:00 to 23:59, I have to insert many null point on each minute point, to reflect the correct ratio on x axis. Finally the for loop will be the bottleneck when drawing, if the date range is a big number

However, If we just calculate the time range, and we can calculate the ratio on x axis, we can simply only render two data, don't have to insert many null values to have the same result.