Start timestamp for graphs inferred from the oldest datapoint

[fbarl]

This is a problem because we say the graphs show 24 hours of data which is not always true.

1. In most cases, the graphs are really showing a little more than 24h of data which is clear from taking a closer look at the time scales

2. What might be a more serious problem is that from the code it seems that we're not really accounting for the missing data at the beginning of the graphs, as nothing similar to History graph x axis should go up to current time #472 has been implemented for the other end of the graph. I couldn't find a zone to demonstrate this case.

[corradio]

Makes total sense!

[Kongkille]

This should actually have been fixed now after we changed the data pïpeline :)