there are minimun two variants:
1. Sort points and create additional tree Plot series for normal, high and low datapoints.
2. Create a special itemRenderer for this. There are many info about creating custom itemRenderers in network. Look for it.
Thank you for your reply.
I am not a pro stilll learning Flex so please let me reiterate
1) I should plit my single series in to three for Hi Lo and Norm
2) How will I use item renderers?
If you have time please shed some more light
For the first variant.
You have a LineSeries for all data.
You manually divide your dataProvider for three providers: with low, hi and norm values. And Create additional PlotSeries for each part and set coloror what you want. So line will be shown by LineSeries and point will be shown and colored by PlotSerieses. Very simple variant but if you need to change the low and hi limits It's not so good. But if this limits are static and data is static it's very good.
If data and limits are dinamic or you want to create more correct solution. It's not so difficult to create itemRenderer for LineSeries (there is a style property itemRenderer). Read about renderers in docs^ http://www.adobe.com/devnet/flex/quickstart/using_item_renderers/