As the attached image from DashBoard>Detailed Usage>Day>Device (in this case my iMac) shows there is one decimal presented i the label that pops up when hovering over one of the green bars. The interesting thing here is that all bars in this screenshot, except the left most one, and the third one from the left, are labelled with 0.1 kWh. The other two are labelled with 0.0 kWh, but their size is not zero! They are clearly showing more detail than that in their size so I think two decimals would be warranted.
Additionally, the bottom of the bars do not touch the 0 value on the y-axis, but I think they should (perhaps just barely, but still).
If no more decimals are added in those labels, perhaps things should be made more consistent by basing the length of the bars on the rounded values shown in the labels, not on the “true” underlying values.