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1. Create a new custom formula field with “Text” formula return type.  
2. Put the following code into the formula syntax:

image("http://chart.apis.google.com/chart?cht=gom&chd=t:&chs=200x75&chco=00FF00,FFFF00,FF8040,FF0000&chxt=x,y&chxl=0:||1:|Recent|Old&chd=t:"&TEXT(Today() - LastActivityDate)&"&chl="& TEXT(Today() - LastActivityDate)&"+Days+Ago","Last Activity")

3. Finally include the same custom formula field on the page layouts.