fix(heatmap): support graceful failure in braille mode (#409)

# Pull Request

## Description
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request. -->
This PR fixes the heatmap's graceful failure bug by changing the braille
input based on the maidr data instead of the visual SVGs

## Related Issues
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addresses. -->
Closes #408 

## Changes Made
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Modified the input to fetch values based on the consistent Maidr data
instead of the inconsistent SVG coordinates

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follows the project's coding standards.
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`ManualTestingProcess.md`, and all tests related to this pull request
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src/js/display.js

@@ -666,13 +666,13 @@ class Display {
       let low = constants.minY + range;
       let medium = low + range;
       let high = medium + range;
-      for (let i = 0; i < plot.y_coord.length; i++) {
-        for (let j = 0; j < plot.x_coord.length; j++) {
-          if (plot.values[i][j] == 0) {
+      for (let i = 0; i < plot.data.length; i++) {
+        for (let j = 0; j < plot.data[i].length; j++) {
+          if (plot.data[i][j] == 0) {
             brailleArray.push('⠀');
-          } else if (plot.values[i][j] <= low) {
+          } else if (plot.data[i][j] <= low) {
             brailleArray.push('⠤');
-          } else if (plot.values[i][j] <= medium) {
+          } else if (plot.data[i][j] <= medium) {
             brailleArray.push('⠒');
           } else {
             brailleArray.push('⠉');