help in selecting all the fields in the list box

[#1]

I am haing a chart in which there is 3 years of data and it consists of

recieved

sales

returns

now i made a chart and everything is good but the customer's all from sales are different and low in number when compared to so now i have to select only sales customers for that i created a seperate list box and now the issue is when I select a push button all the customers in list box should select at a time.

[Anonymous]

create a button and in action --->slect posible -----> give the field name which you want all values ned to be selected