Fixes Line chart zero filling issue.

Closes #2102.

The problem re-occurred due to the fact that I made the assumption that we were only dealing with visualizations with 1 chart. So, when multiple charts were added, zero-filling was again broken.

The previous problem was that x axis values were not being sorted by date for time based visualizations. Which made lines in line charts appear out of order. To fix this, we added an isDate parameter to each object when determining x axis value order. I assumed that the `ordered` object would be at the vis level in the data that the function received. However the `ordered` object always occurs at the chart level. So, we needed to add code that checked each chart data for an `ordered` object.

Tests still need to be written for this, but there is already an open issue for that. #2166.