The Watt (W) is a measurement of power, the rate at which you consume energy. Energy is thus measured in Wh, so in 10 hours a 100W lightbulb (if there are any left) would use 10 x 100 =  1000 Wh = 1kWh (k, kilo, indicates 1000).

Your usage histograms show energy consumption in kWh. If you have used 0.46 kWh in half an hour, the average power of the things using electricity during that half hour is 2 x 0.46 = 0.92 kW or 920 W. You may see this referred to as the power draw : “My hoover draws 1200 W at full blast” (you should see the power rating of every electrical item on the item itself). If I ran my hoover for half an hour, it would consume ½ h x 1200 W = 600 Wh = 0.6 kWh. 

Your meter measures the amount of energy you use every half hour, so you can add all those figures up for the whole day to see approximately how much you used that day. That is what OVO bill you for.

My point is that they are indicating time based values taken at known time intervals. If the interval was 1 hour then kW = kWh anyway.

It would be much clearer to label interval usage as kW.

See if this helps

https://www.ovoenergy.com/guides/energy-guides/what-is-a-kwh-kw-and-kwh-explained

If it doesn't help I will find you some more links from other suppliers which explain it as well. 

I’m not sure I understand what you’re suggesting. Here’s my usage histogram for yesterday morning:
  

This tells me very clearly that I wasn’t using much energy at all until 07:30, when I used 100Wh making a pot of tea (costing me about 4½p). However, the shower I took at 08:30 used up 2kWh, which I reckon cost me about 90p. This makes me think I spent about twice as long in the shower as I should have done. I also see that the kettle, which is rated at 3kW, used about as much energy boiling 1l of water as the toaster - rated at 1.5kW - did to make three slices of toast at 09:00. 

How would you prefer to see these data represented? How would showing kW values help me make sense of my energy consumption? 

Thinking a bit more about your suggestion, I recalled that at least one of the third-party apps available to retrieve and display smart meter data has an hourly histogram setting:
  

There is no indication on the ‘y’ axis of the units displayed, so you could regard them as kW indicating the average power draw in each hour. I still can’t see that this is any clearer than the more granular half-hourly chart, and it could even be misleading.