Next and Prev buttons work in reverse

I have noticed that when I view graphs of my data, e.g. Weekly generation data, the graph is shown correctly, but if I want to view a previous week, I have to click on the Next button, and vice versa. I wondered whether it was affected by the up/down arrow in the head of the data table, but that only influences the order of the bars in the graph; the buttons still work backwards to my mind.

Any ideas?

The graph data is shown left → right which corresponds to the table data bottom → top, the ‘next’ button retrieves the next set of data based on the current ordering.