This post walkthrough 'creating a mobile trend graph from your own data' -- Start by creating advanced segment. Go to reports in Google Analytics. In left navigation, a grey box My Customizations appear, click Advanced Segments. On next screen, click Create new custom segment. This take you to segment builder screen. Under Dimensions (in green), expand Systems and drag "Operating System" into work area and create this definition:
Operating System Matches Exactly Android
Now, add an OR condition and add another definition:
Operating System Matches Exactly iPhone
Keep adding as many OR conditions and definitions as you like.
Name segment "mobile phones" or any and save it. Use now, Advanced Segments dropdown to apply segment. To see trend graph, go to any report that graphs visits, Dashboard will work fine for this. Set your date range to include last 18 months or 2 years. The trend will be most clear if you select Graph by Month icon (at top right of graph).
Advanced Segments in Google Analytics: