ETA plot overall mean

[milesOIST]

When trying to visualise ETA, the Select Item drop down is called "mean", but when you select this option, it is not possible to plot a for all cells. Currently it makes a new plot for each ROI, averaging over all events. However, it would be useful to plot the average for all ROIs for all events. This is the most common type of plot one sees in a paper.
It would be great to select different types of error to plot as well (STD and SEM are the most common).