the chart is the IV on the underlying stock, not individual options. It says it right under the chart. I dont know how they are calculated, taking the average or atm strikes? But the number isnt that important, the chart shows you the IV history and comparison vs historical that's important.
To get IV on individual options, click on the detail link next to it, then it will show you all the greeks and IV number.
If you really want to know how they calculate the chart iv, send an email to support. Optionsxpress has one of the best cs, they will answer it in a day or 2. I have got all my technical questions answered in great detail from their support.