Fairly easy solution to stumble upon, but just in case people don't know..
I've been wanting to create a bar chart with negative values in Google Data Studio. Creating the default chart would result in the following:

It uses 0 as the y-axis minimum (everything under Bar Chart Properties > Style), so the above is expected. You'd think the simple fix would be to just lower the y-axis to a negative value, but it then displays like so:

So the bars now display from -infinity. The simple fix? Delete the y-axis min value so it becomes (auto). Now it looks like so:

You lose some flexibility like if you want to put the axis in the middle of the chart. Data Studio still in beta after all.