contribution-vs-interest

spec.json

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "Plots two functions using a generated sequence.",
  "width": 600,
  "height": 300,
  "title": "Figure out what monthly contribution is equivalent to what savings rate",
  "data": [{
    "name": "month",
    "values": [

    ],
    "transform": [
      {"type": "sequence", "start": 0, "stop": 200, "step": 10, "as": "c"},
      {
        "type": "formula",
        "expr": "datum.c*months+balance",
        "as": "v"
      },
      {
        "type": "formula",
        "expr": "(pow(datum.v / balance, 1/months)-1)*12*100",
        "as": "y"
      },
      {
        "type": "formula",
        "expr": "1.1",
        "as": "comparison"
      },
      {
        "type": "formula",
        "expr": "datum.y <= 10 ? true : false",
        "as": "defined"
      },
            {
        "type": "formula",
        "expr": "datum.y <= 10 ? datum.y : 10",
        "as": "y"
      }
    ]
  }],
  "signals": [
    {
      "name": "balance",
      "value": 1000,
      "description": "How much money you start with",
      "bind": {"input": "range", "min": 10, "max": 20000, "step": 1, "debounce": 10}
    },
    {
      "name": "months",
      "value": 60,
      "description": "How many months you save for",
      "bind": {"input": "range", "min": 1, "max": 240, "step": 1, "debounce": 10}
    }
  ],
  "scales": [
    {
      "name": "x",
      "type": "point",
      "range": "width",
      "domain": {"data": "month", "field": "c"}
    },
    {
      "name": "y",
      "type": "linear",
      "range": "height",
      "nice": true,
      "zero": true,
      "domain": {"data": "month", "field": "y"},
      "domainMax": 10
    }
  ],

  "axes": [
    {"orient": "bottom", "scale": "x", "title": "Monthly Contribution ($)"},
    {"orient": "left", "scale": "y", "title": "Equivalent Interest Rate (% p.a.)"}
  ],

      "marks": [
        {
          "type": "line",
          "from": {"data": "month"},
          "stroke": "red",
          "encode": {
            "update": {
              "x": {"scale": "x", "field": "c"},
              "y": {"scale": "y", "field": "y"},
              "strokeWidth": {"value": 2},
              "defined": {"field": "defined"}
            }
          }
        }
      ]
}