Thank you for your advice. I looked up the persistent Colors option and found out it was an option to keep color once we make change in selection.
However what I need is to highlighten the chart I click on. Just like when you click on a file on your machine, the file's background will go blue. I am wondering whether Qlik can do this.
What you are describing is commonly called "Brushing", and yes, it's possible with Qlikview. Download this sample for some examples:
Qlikview Cookbook: Focus not Filter http://qlikviewcookbook.com/recipes/download-info/focus-not-filter/