Is there a way to upload data retrospectively into HA Energy Dashboard?
My energy provider enables half hourly data to be downloaded on a daily basis, and I wonder is there is any way of inserting this into the Energy Dashboard.
It would also be good if the dashboard could handle other utility data (such as water and gas) in the same way (as per the feature request).
Yes, I can upload the data to different databases and use it that way, but wondered if this was possible or being considered.
No. Real-time additions to the recorder database only.
I’ve seen some talk of working out a way to add historical data. It was just a passing comment though. So the devs are aware some people want this but I’m not sure if it is actively being worked on.
As has been previously explained, the energy dashboard is for energy. What you (and the feature request) are asking for is a general utilities dashboard. This is not currently being considered, but is very easy to make yourself.
Gas is energy!
Perhaps it should be renamed - Electricity Dashboard 
Sure, if its unit is kWh you will find that is already included. However you said:
Water is not Energy. Unless you were talking about micro hydro (which I strongly suspect you were not). Then its unit too will be in kWh and will be available in the energy dashboard.
My HA energy dashboard provides (based on the smart meter info) an overview of Kwh (electricity) and m3 (Gas) used 
Apologies I thought it had to be in kWh (I don’t use it).
Water still does not belong there.
Interesting
So if you did your water as m³ instead of litres, that would work as well 
- not such a big leap.
This would need a Flow/Volume device class as well