I was palying with bubble charts and I found that they would not display if the data had the X axis values as strings. They work fine if the values are numbers. See the options below. I can't think of a reason for this. Any ideas?

this works:
{
series: [
{
type: 'bubble',
data: [[1, 40, 60], [2, 30, 40], [3, 0, 30], [4, 30, 35]]
}
]
}

this doesn't work:
{
series: [
{
type: 'bubble',
data: [['A', 40, 60], ['B', 30, 40], ['C', 0, 30], ['D', 30, 35]]
}
]
}

I am using jqChart v3.8.5.0.