Provide a default implementation for the case of only one float series

[marianogappa]

There's a very common case for input that looks like this:

1
5
10
11
12
...

It's not clear what chart should do in this case. The two most likely charts that one wants in this case (in my opinion) are:

1. histogram (most likely)
2. line chart with the data series being the y-axis, and with an implicit monotonically increasing x-axis (i.e. want to see the evolution of the data series line by line, e.g. the evolution of a stock price every hour)

At the moment, however, chart just returns: FATA[0000] couldn't find values to plot.

Given that we don't support histograms, 2) should be supported instead.