Negative memory usage

For some reason, I get a negative memory usage. In the screenshot below -6136 Mb, clearly this is impossible.

For comparison, free -h returns the following.

              total        used        free      shared  buff/cache   available
Mem:           7.7G        643M        120M         38M        6.9G        6.7G
Swap:          7.9G        844K        7.9G

I am having trouble finding the relation between both. Is it also possible that the graph is shown in megabytes (MB) instead of megabits (Mb)?

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