problems with reading unit as a string

Hi

I am pretty new to this "technology" and I am not able to read an xml file with string in the the unit section.
I am able to load a file with a max of 3 string data but as I put a fourth data the script returns:

"JSChart XML: File not loaded or malformed color/data/option set"

any suggestion is very welcome .

thanks

ps:

this file works:

<?xml version="1.0"?>
<JSChart>

<dataset type="line">
<data unit= 'unit1' value="15.4"/>
<data unit= 'unit2' value="2.2"/>
<data unit= 'unit3' value="2.2"/>
</dataset>

</JSChart>


this doesn't


<?xml version="1.0"?>
<JSChart>

<dataset type="line">
<data unit= 'unit1' value="15.4"/>
<data unit= 'unit2' value="2.2"/>
<data unit= 'unit3' value="2.2"/>
<data unit= 'unit4' value="2.2"/>

</dataset>

</JSChart>