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dont plot null values #104

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Merged
merged 3 commits into from
Mar 8, 2025
Merged

dont plot null values #104

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3788e9d
dont plot null values at all (was plotting as zeros)
melange396 Mar 8, 2025
ff86130
dont plot null values at all (was plotting as zeros) -- part 2
melange396 Mar 8, 2025
f72a3a9
use correct 'or' operator
melange396 Mar 8, 2025
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5 changes: 3 additions & 2 deletions src/components/Chart.svelte
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Original file line number Diff line number Diff line change
Expand Up @@ -719,10 +719,11 @@
ctx.fillStyle = color;
ctx.beginPath();
for (let i = 0; i < data.length; i++) {
const y = value2y(dataset.getPointValue(i));
if (isNaN(y)) {
const ptVal = dataset.getPointValue(i);
if (isNaN(ptVal) || ptVal == null) {
continue;
Comment on lines -722 to 724
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@melange396 melange396 Mar 8, 2025

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This looks like it was intended to prevent plotting null values in the data, but there were two major problems with the logic:

First, JavaScript's isNaN() doesnt quite determine if the object in question is a number or not; it determines if it is (or if it would be coerced to) the numeric value of NaN (a reserved word in the language). You might think that null is not a number, but when used in a numeric expression, null is "coerced" to the numeric value of zero. (Similarly, the string "1" is coerced to numeric 1 and string "0" to numeric 0. The string "xyz" would be coerced to NaN because it cannot be interpreted as a numeric value. For strange historical reasons and by convention, the empty string "" also becomes 0.) Thus, isNaN() is not sufficient to test for null, and that must be done explicitly.

isNaN(null)  == false;
isNaN(0)     == false;
isNaN("1")   == false;
isNaN("")    == false;
isNaN(NaN)   == true;
isNaN("xyz") == true;

Second, since null is coerced to 0 when used in a numeric expression, we must test a value for equality to null before it is run through such an expression... And it is transformed by arithmetic operations when passed to value2y(), so testing for null afterward is futile!

let a = null, b = null;
a += 0; // identity operation?
b *= 1; // identity operation?
console.log(a, b); // => "null null"

}
const y = value2y(ptVal);
date1 = date2;
date2 = data[i].getDate().getIndex();
if (date2 - date1 !== gap) {
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2 changes: 1 addition & 1 deletion src/data/DataSet.ts
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Expand Up @@ -65,7 +65,7 @@ export default class DataSet {

getPointValue(index: number): number {
const temp = this.data[index].getValue();
if (Number.isNaN(temp)) {
if (Number.isNaN(temp) || temp == null) {
return temp;
}
return this.verticalOffset + temp * this.scale;
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