Problem with automatic legend with grdvector

Hello,

I am using the grdvector to represent ocean currents and would like to include a reference vector in the legend. I uploaded an example dataset I am using to: https://drive.google.com/file/d/1rrYrVIQMPHJl868BMZrr7Nqkhq6QXKVw/view?usp=sharing

The file contains Cartesian vectors with the eastward (ugos) and northward velocity components. To use the automatic labeling of GMT, I transformed Cartesian to polar coordinates. Finally the code that I used to represent the vector is:

```
grdvector(Gr, Gtheta, polar=true, spacing=0.5,
vec_scale=(scale="4.5c", scale_at_lat=true, refsize=1.),
arrow=(shape=:arrow, stop=true, pen=("20p,black",),
len=0.16, fill=:black, norm=2.), legend=(label="1 m/s", pos=:MiddleCenter))
````

However I obtained the following error:

```
grdvector [ERROR]: Option -S : No vector scale point is allowed for Cartesian vector
```

Could you help me to identify what I missing?

Thank you