It plots the total number of suspected, probable and laboratory-confirmed cases between reports - which is a measure of change over time that is not cumulative.
That's not to say that understanding this chart is easy for everyone...as with everything, what you take away from it may be heavily influenced by your own perspective and your background in reading graphs. I have written something about how to read some of the graphs on my blog here, which may be helpful too.
|Uses World Health Organization data up to and including the Situation Report from the 5th-Sept, 2014.|
Click on chart to enlarge.
You can also mouse over the dots on the interactive version of the graph here. That will tell you the dates. THe subtraction is up to you though!
The lines joining the dots here suggest what is happening between the WHO Reports, but the line do not actually use any real collected values...because we don't have them to plot.
Technically, a bar graph would be more accurate, but I find a line graph easier to read at a glance. So do remember - we don't know what is happening between those dots. We're just presuming it.