add ability to draw arbitrary lines

sometimes you want ability to draw lines to make it seem like you
know what you're doing and that the market behaves rationally.

app/api/v1/data.py

@@ -53,7 +53,6 @@ def sales_data():
 
 @bp.get('/data/deaths')
 def death_data():
-    # https://www150.statcan.gc.ca/n1/daily-quotidien/231127/t001b-eng.htm
     table = pa_csv.read_csv('app/static/data/can-deaths.csv')
     data = {'data': table.to_pylist()}
     if request.headers.get('Hx-Request'):

app/static/js/utils.js

@@ -27,3 +27,128 @@ function addData(chart, newData, labels) {
   }
   chart.update();
 }
+
+const chartStates = new WeakMap();
+const arbLines = {
+  id: "chartjs-arb-lines",
+
+  defaults: {
+    color: "black",
+    lineThreshold: 15,
+    lineWidth: 2,
+    enableKey: "Control", // draw only when keypressed
+    modifierKey: "Shift", // extends line to borders
+  },
+
+  beforeInit(chart) {
+    chartStates.set(chart, {
+      lines: [],
+    });
+  },
+
+  afterEvent(chart, args, options) {
+    const { ctx, chartArea } = chart;
+    const state = chartStates.get(chart);
+
+    switch (args.event.type) {
+      case "mousedown":
+        if (args.replay !== true && isKeyDown(options.enableKey)) {
+          // registers mousedown event twice if you click (don't move mouse), ignore one.
+          state.startXY = { x: args.event.x, y: args.event.y };
+        }
+        break;
+      case "mousemove":
+        if (state.startXY) {
+          ctx.beginPath();
+          ctx.lineWidth = options.lineWidth;
+          const line = getCoords(chartArea, {
+            ...state.startXY,
+            x2: args.event.x,
+            y2: args.event.y,
+            full: isKeyDown(options.modifierKey),
+          });
+          ctx.moveTo(line.x1, line.y1);
+          ctx.lineTo(line.x2, line.y2);
+          ctx.strokeStyle = "grey";
+          ctx.stroke();
+          ctx.restore();
+        }
+        break;
+      case "mouseup":
+        if (
+          isKeyDown(options.enableKey) &&
+          Math.abs(state.startXY.x - args.event.x) + Math.abs(state.startXY.y - args.event.y) >
+            options.lineThreshold
+        ) {
+          // don't draw tiny lines
+          state.lines.push({
+            ...state.startXY,
+            x2: args.event.x,
+            y2: args.event.y,
+            full: isKeyDown(options.modifierKey),
+          });
+        }
+        state.startXY = null;
+        break;
+    }
+  },
+
+  afterDatasetsDraw(chart, args, options) {
+    const { ctx, chartArea } = chart;
+    const state = chartStates.get(chart);
+    for (const line of state.lines) {
+      ctx.beginPath();
+      ctx.lineWidth = options.lineWidth;
+      const drawLine = getCoords(chartArea, line);
+      ctx.moveTo(drawLine.x1, drawLine.y1);
+      ctx.lineTo(drawLine.x2, drawLine.y2);
+      ctx.strokeStyle = options.color;
+      ctx.closePath();
+      ctx.stroke();
+    }
+  },
+};
+
+function getCoords(chartArea, line) {
+  if (line.full === false) {
+    return {
+      x1: line.x,
+      y1: line.y,
+      x2: line.x2,
+      y2: line.y2,
+    };
+  }
+  // NOTE: i graduated highschool and i had to google how to y = mx + b ...
+  const slope = (line.y - line.y2) / (line.x - line.x2);
+  const intercept = line.y - slope * line.x;
+  let x1 = chartArea.left;
+  if (slope * chartArea.left + intercept < chartArea.top) {
+    x1 = (chartArea.top - intercept) / slope;
+  } else if (slope * chartArea.left + intercept > chartArea.bottom) {
+    x1 = (chartArea.bottom - intercept) / slope;
+  }
+  let x2 = chartArea.right;
+  if (slope * chartArea.right + intercept < chartArea.top) {
+    x2 = (chartArea.top - intercept) / slope;
+  } else if (slope * chartArea.right + intercept > chartArea.bottom) {
+    x2 = (chartArea.bottom - intercept) / slope;
+  }
+  return {
+    x1: x1,
+    y1: Math.min(Math.max(slope * x1 + intercept, chartArea.top), chartArea.bottom),
+    x2: x2,
+    y2: Math.min(Math.max(slope * x2 + intercept, chartArea.top), chartArea.bottom),
+  };
+}
+
+const isKeyDown = (() => {
+  // https://stackoverflow.com/a/48750898
+  const state = {};
+
+  // biome-ignore lint: let it mod
+  window.addEventListener("keyup", (e) => (state[e.key] = false));
+  // biome-ignore lint: let it mod
+  window.addEventListener("keydown", (e) => (state[e.key] = true));
+
+  return (key) => (Object.hasOwn(state, key) && state[key]) || false;
+})();

app/templates/analytics.html

@@ -35,7 +35,7 @@ <h1 class="title">
       </div>
       <canvas hx-get="/api/v1/data/sales" hx-trigger="load" hx-indicator='.htmx-indicator'
         id="line_chart_id"></canvas>
-      <span>https://data.bts.gov/Research-and-Statistics/Auto-Sales/7n6a-n5tz</span>
+      <sup>source: https://data.bts.gov/Research-and-Statistics/Auto-Sales/7n6a-n5tz</sup>
       <progress class="progress is-small htmx-indicator" max="100"></progress>
     </div>
   </section>
@@ -44,7 +44,7 @@ <h1 class="title">
     <div class="columns">
       <div class="column">
         <div class="field">
-          <button class="button" hx-get="/api/v1/data/random" hx-trigger="click, revealed"
+          <button class="button" hx-get="/api/v1/data/random" hx-trigger="click, load"
             hx-swap="none">populate</button>
         </div>
       </div>
@@ -57,6 +57,7 @@ <h1 class="title">
   <section class="section">
     <div class="container">
       <div class="is-striped" id="death-table" hx-get="/api/v1/data/deaths" hx-trigger="revealed"></div>
+      <sup>source: https://www150.statcan.gc.ca/n1/daily-quotidien/231127/t001b-eng.htm</sup>
     </div>
   </section>
 
@@ -92,7 +93,9 @@ <h1 class="title">
       document.getElementById("line_chart_id"),
       {
         type: "line",
+        plugins: [arbLines],
         options: {
+          events: ['mousedown', 'mouseup', 'mousemove'],
           responsive: true,
           plugins: {
             legend: {