How to understand disk activity view charts of the Windows 7 resource monitor?

The disk activity monitor charts show blue line, green line with the green shaded area below and sometimes also the orange line. Of course, the higher the lines the more the activity and horizontal axis is clearly the time but what is exactly plotted there? I have looked into disk activity monitor help but did not find the legend of these charts there.

I mean, Resource Monitor, Disk tab, charts on the right.

UPDATE:

Many thanks to RoG below for more clarification on this answer:

Green is data being WRITTEN TO and READ FROM the drive.

Blue shows a statistical quantifier of the amount of data that has been written to and from from the drive, as explained by RoG below:

"...Blue...showing the current value of the green line divided by the maximum value of the green line, as a percentage."

Orange shows the impact of a specific task, as chosen by checking a box to highlight the task on the left of the window.

According to Microsoft TechNet:

Disk displays the total current I/O in green, and displays the highest active time percentage in blue.

Notice that this corresponds to the Disk Activity list on the left (might need to expand it), which shows "XX KB/sec Disk I/O" and "XX% Highest Active Time". So, I interpret this as green being both reads and writes, and blue also being reads and writes, but showing the current value of the green line divided by the maximum value of the green line, as a percentage.

Admittedly my source is for Windows Server 2008, not Windows 7.

The orange line shows I/O for a specific task, enabled by checking a box in the "Processes with Disk Activity" list on the left (might need to expand it also).