LineChart trouble

I'm trying to display a relatively simple line chart, but for some reason I
can't get the line to show up. I have an ArrayCollection of value objects,
and here are the relevant items:

CowsFormModel{
var Datestamp:Date;
var TotalScore:int;

...
//set dataProvider
_page.cowsChart.dataProvider = _page.cowsData;//Contains ArrayCollection of
CowsFormModel items
...
<mx:LineChart id="cowsChart" width="100%" height="100%"
showDataTips="true"
fontFamily="Arial" fontSize="13" clipContent="false">
<mx:verticalAxis>
<mx:LinearAxis id="cowsVAxis" baseAtZero="true" maximum="48"/>
</mx:verticalAxis>
<mx:horizontalAxis>
<mx:DateTimeAxis dataUnits="days" />
</mx:horizontalAxis>
<mx:series>
<mx:LineSeries xField="Datestamp" yField="TotalScore" displayName="Total
Score" />
</mx:series>
</mx:LineChart>
<mx:Legend width="100%" direction="horizontal" dataProvider="{cowsChart}"
fontFamily="Arial" fontSize="13" />
...
I know that there are two items in the array with TotalScore as 15 and 13,
but this is all that is displayed
<http://apache-flex-users.2333346.n4.nabble.com/file/n9309/Screen_Shot_2014-12-31_at_10.png>