dont plot null values

[melange396]

This will not plot null values at all, whereas before it was showing them as "zero" in the y dimension.

This effort is in furtherance of issue:

• Figure out how best to plot NULL values #91

[melange396]

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"

[RoniRos]

LGTM.