Hi
You would need to include the months in the dataProvider array, but set their values to "" instead of 1000 for example. The code below shows this working if you run it:
<?xml version="1.0"?>
<!-- charts/BasicLine.mxml -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Script><![CDATA[
import mx.collections.ArrayCollection;
[Bindable]
public var expenses:ArrayCollection = new ArrayCollection([
{Month:"Jan", Profit:2000},
{Month:"Feb", Profit:1000},
{Month:"Mar", Profit:""}
]);
]]></mx:Script>
<mx:Panel title="Line Chart">
<mx:LineChart id="myChart"
dataProvider="{expenses}"
showDataTips="true"
>
<mx:horizontalAxis>
<mx:CategoryAxis
dataProvider="{expenses}"
categoryField="Month"
/>
</mx:horizontalAxis>
<mx:series>
<mx:LineSeries
yField="Profit"
displayName="Profit"
/>
</mx:series>
</mx:LineChart>
<mx:Legend dataProvider="{myChart}"/>
</mx:Panel>
</mx:Application>