Hey @momo1104
You could separate the rule marks into their own layer. The difficulty with this is determining a pixel length/height for each line to get say a 20px x 20 px cross with the origin at the mean/mean coordinate. See example here as shown below.
Another approach would be to use a point mark instead, with a simple svg cross path scaled to whatever pixel value you want. The layer would look something like this...
{
"mark": {
"type": "point",
"shape": "M -1 0 L 1 0 M 0 1 L 0 -1",
"strokeWidth": 1,
"size": 10000,
"color": "red"
},
"encoding": {
"x": {
"field": "certainty",
"type": "quantitative",
"aggregate": "mean"
},
"y": {
"field": "threat",
"type": "quantitative",
"aggregate": "mean"
}
}
}
And the chart would look like this...
See the full example here.