R3T is a HP only internal tool. As above for now you can export the data to a .CSV file and forecast in MS Excel.
Here's a quick example of trending with exported data.
1. Go to the Performance and Reports section of IMC
2. Select New Report, Physical Disks, Space, fill in the period you're interested in and generate the report.
3. Click the Export data button and select CSV Format & File to save the data to a CSV.
4. Open up Excel and load the file you just saved.
5. Select the data you're interested in e.g the "Normal Used OK" column, these are chunklets BTW.
6. Now insert a new line chart
7. Once you have the chart, right click the "Normal Used OK" line and select Add Trendline
8. Select the trendline type and the number of periods you would like to look forward e.g 90 days
Takes about 2 minutes in all and you can save the report along the way for reuse on your next run.
Here's what it looks like
https://db.tt/RDsRRXeV the blue line being the data and the red line being your forecasted growth. You can play about exporting or graphing other values to meet your requirement, the above is intended as a quick example. Just keep in mind the forecast is only as good as the data you supply, so you'll want to look at a relatively steady state over a long period of time.