Chart Lengend not showing properly

I have a line chart that is working okay, and it shows the legend, but the legend only shows a red square and a blue diamond, it doesn't label what these items are supposed to be (see attachment.) Can someone assist in this bug?

Here is the chart script:

chart  SQUD4242N2_Usage
{
    displayname = "42x42 2-Way Usage"
    type = XYLINE
    show  all  rows  from  Main 
    (
        x-axis
        (
            title = "Date"
            field = Date_field
        )
        y-axis
        (
            title = "Usage"
            fields = Usage_P1_SQUD4242N2, Usage_P2_SQUD4242N2
        )
    )
    filters 
    (
        Date_field
    )
    sort by
    (
        Date_field   ascending
    )
    options
    (
        width  = 1200
    )
}