Dynamically change color of vega-lite area plot based on result of linear regression

We have time series data with an overlay of the linear regression to indicate the trend

Now we would like to color it red if the trend is declining as shown here

See the example

We have tried various ways with parameters but did not manage to have the color dependent on the trend. Our approach was focused on getting the slope and check if it was positive or negative.

Here you go. I'll let you fix the gradient.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "params": [
    {
      "name": "slope",
      "expr": " (data('data_1')[1]['date']-data('data_1')[0]['date'])/(data('data_1')[1]['value']-data('data_1')[0]['value']) "
    },
    {
      "name": "red",
      "expr": "{gradient: 'linear', stops: [{offset: 0, color: 'red'}, {offset: 1, color: 'red'}]}"
    },
    {
      "name": "green",
      "expr": "{gradient: 'linear', stops: [{offset: 0, color: 'green'}, {offset: 1, color: 'green'}]}"
    }
  ],
  "data": {
    "values": [
      {"date": "2023-03-31", "value": 80},
      {"date": "2023-06-30", "value": 78},
      {"date": "2023-09-31", "value": 81},
      {"date": "2023-12-31", "value": 75},
      {"date": "2024-03-31", "value": 72},
      {"date": "2024-06-30", "value": 75},
      {"date": "2024-09-31", "value": 70}
    ]
  },
  "encoding": {
    "x": {"field": "date", "type": "temporal"},
    "y": {"field": "value", "type": "quantitative"}
  },
  "layer": [
    {
      "mark": {
        "type": "line",
        "color": "rgba(0,0,0,0.5)",
        "strokeWidth": 2,
        "clip": true
      },
      "transform": [{"regression": "value", "on": "date"}]
    },
    {
      "mark": {
        "type": "area",
        "clip": true,
        "line": {"color": {"expr": "slope>0?'red':'green'"}, "strokeWidth": 4},
        "color": {"expr": "slope>0?red:green"}
      }
    }
  ]
}