Accessing regression params with groupby

[ghost]

Hello,
I'm trying to plot 2 regression lines with their corresponding r2 values using groupby. I can get the lines to appear on the plot for the 2 groups but I'm not sure how to access the rSquared values for the 2 groups.
Code I have right now:
{
"regression": "y_column_name",
"on": "x_column_name",
"params": true,
"groupby": [
"groupby_column_name"
]
},
{
"calculate": "'R2:'+ format(datum.rSquared,'.2f')",
"as": "r2"
}

How do I modify datum.rSquared to access the 2 groups' values?

Thanks!