The top area bar graph is the volume of transactions processed for a given day. The bottom line graph is the average processing time for that day. Does it make sense to combine them together into a single graph (i.e., the line graph overlaid on the bar graph)? Is there any good way to display them together as a single graph without the data being too convoluted?
Does it make sense to combine them together into a single graph (i.e., the line graph overlaid on the bar graph)?
Usually not. Each overlaid graph will be harder to understand. The reader will have to figure out which axis goes with which element and ignore coincidental features of the combination. Use it only if space is extremely limited and worth more than comprehension.
If one variable is secondary to the other you might show that variable with an attribute such as color. For instance, here is a graph of the times trend using color to indicate volume. This might be useful if you wanted to examine an interaction between all three variables, such as "in August, above average transaction times corresponded with above average volumes" which would be represented by dots above the trend line being redder in that period.