When bbox_to_anchor and loc are used together, the loc argument will inform matplotlib which part of the bounding box of the legend should be placed at the arguments of bbox_to_anchor. For example (I’ve simplified the command a bit), the three options below will produce different locations for your legend,
fig.legend([line1], ['series1'], bbox_to_anchor=[0.5, 0.5], loc="center")
fig.legend([line1], ['series1'], bbox_to_anchor=[0.5, 0.5], loc="center left")
fig.legend([line1], ['series1'], bbox_to_anchor=[0.5, 0.5], loc="center right")
The first command will put the center of the bounding box at axes coordinates 0.5,0.5. The second will put the center left edge of the bounding box at the same coordinates (i.e. shift the legend to the right). Finally, the third option will put the center right edge of the bounding box at the coordinates (i.e. shift the legend to the left).