How can I calculate 3 month avg and display in chart

i have a datepicker on two edittext to get a value of startdate and enddate. value can get esaly now a chart have not display 3 months value in grahp properly . value was store in datamodel. so i have to calculate a average of 3 month and then display. how can i calculate a average of that string value. plzzz help me i use mpandroidchart to display line chart.