Column chart with null data items

Question

I have a Column Chart with an ArrayCollection data provider.
The data will sometimes have missing items in some "rows'. For
example, it might look like this where the second object has no
"Male" property:
public var myData:ArrayCollection = new ArrayCollection([
{School: "Priorford", All: 95, Male: 92, Female: 98},
{School: "Giffnock", All: 87, Female: 89},
{School: "Hastings", All: 80, Male: 78, Female: 82}
Sometimes I get the following error:
TypeError: Error #1009: Cannot access a property or method of
a null object
reference.
at mx.charts.chartClasses::NumericAxis/mapCache()
I'm assuming the error is because of missing data items, but
I'm not 100% sure. Some data sets with missing items will display
with no error. I can't see a pattern to those that don't.
-- I can't produce the error when using a "static" data
source set in the Flex app itself (such as the example above).
These always display correctly, even with missing items.
-- In the problem case, the source data comes from a MySQL
database through a PHP script that is called with a Flex
httpService. The returned data is parsed in Flex, and put into the
ArrayCollection. I was suspecting that the returned data was flawed
in some way, but the same ArrayCollection displays correctly in a
DataGrid. The problem is only with the Chart.
-- I can work round the problem by adding the missing
properties and setting them to an empty value. But I don't think I
should have to do this.
Has anyone seen this problem, or know of any obvious thing I
might be doing wrong.

Answer 1

Thanks Arthur.
Your first suggestion is effectively what I'm doing as a
workaround. But it is a bit of a pain to have to account for this
each time when displaying a chart. It's a particular problem when
you don't know in advance what "categories" might be returned by
the data and that have to be charted.
My biggest concern is that this looks like a bug, and the
fact that it seems to occur intermittently is a bit worrying. (I
also know that eight out of ten reported "bugs" are user error,
which is why I was wondering if anyone else had experienced this.)
About your second suggestion, I think the interpolateValues
property only applies to lineSeries.