Fix gas cost chart when there is a single meter

[ldodds]

Testing #4123 I noticed a long standing issue with the gas costs chart.

The issue only affects schools with a single gas meter and only on the 2 year comparison chart.

The cost pages use charts named with an electricity_ prefix. The default definition of that is to show the electricity costs. However the charts override that by specifying a meter (either a real gas/electricity meter or the aggregate one), which means the charts then shows the costs for that meter.

When a school has a single meter, the 2 year comparison chart was not specifying a meter, so it was defaulting to the electricity costs.

I've fixed this by updating the partials to build a chart_config for the aggregate (gas or electricity) meter so the right values are used.

The naming is a hold-over from previous versions of these pages and ideally needs tidying up. We should do that as a separate exercise, but there's a lot of other chart configurations to tidy as well.

[ldodds]

@tbhi do you mind reviewing this for today's release?

[tbhi]

surprised no erblint complaints