[FEATURE] Automatic adjustment of yAxes range based on displayed data

[IvanRF]

Demonstration: http://codepen.io/anon/pen/jBgrGR

The dataset includes a high value for a past date: ('2017-01-01', 1200) and lower values for the Last 7 days.

When the view only shows the last 7 days by changing time.min and time.max on the x axis, the high value (1200) corresponding to a date outside the view makes the entire chart unreadable. That is, the max value of the y axis should be automatically updated based on the data being displayed. In this example, 1200 should be updated to 7, which is the highest value for the last 7 days.

Could this behaviour be added as an option for the chart? or did miss something already available?

[etimberg]

@IvanRF supporting this will take some work inside the chart. I'm happy to look at some kind of PR / proposal for this feature. The data limits for a linear axis are determined here: https://github.com/chartjs/Chart.js/blob/master/src/scales/scale.linear.js#L16-L132 and for a logarithmic axis here: https://github.com/chartjs/Chart.js/blob/master/src/scales/scale.logarithmic.js#L17-L135

Maybe some common logic could be factored out at the same time.