HTML in Spotfire and IronPython
What you can do is add a new text area. Then, add a drop down list in this text area. You can create a new Document.Properties linked to this drop down list.
You can create many options in your drop down list, and your Document.Properties will have the selected value of your drop down list.
Text Area could be designed in HTML/CC so you can customize as you want.
Once you have your Document.Properties with the drop down list value, you can go on your chart properties and add a custom expression with your Document.Properties like [MyCol]==$(docproperties) 
I hope it will help you !
I want to make a drop down list in spotfire using html. So based on choice selected I want to show custom divs. How to implement that ?? Can anyone help with the ironpython script