Several widgets have the ability to represent threshold lines through the options attributes.

Those thresholds have to be configured setting a color and a label as description.

{
  "thresholds": [
    { "value": 200000000, "color": "red", "label": "Threshold 1" },
    { "value": 100000000, "color": "#000000", "label": "Threshold 2" }
  ]
}