Loading in new data may completely break the graph

[SpyTec]

• C3 version: 0.7.3
• D3 version: 5.9.7
• Browser: Firefox 68
• OS: Fedora 30

Upon loading some data, then changing the dataset completely (with the same series data), it may render it completely unusable or render a dataset that doesn't exist (rendering a dataset that doesn't exist happens everytime when unload arg in load is set).

I have no idea what is causing it. Might be related to #2673

Some examples:

When graph is created: Displaying dataset 1

Switching from dataset 1 to dataset 2. Renders it completely unusable

When graph is created: Displaying dataset 2

Switching from dataset 2 to dataset 1

[SpyTec]

Notice how switching from dataset 2 to 1 doesn't update the x-axis either (despite axis.x.categories being included in load()). The end-date is set first before the data is loaded in, but the data doesn't update correctly.