set to null instead of delete in ohlc/calc

Author: alexcjohnson

src/traces/ohlc/calc.js

@@ -19,9 +19,9 @@ function calc(gd, trace) {
 
     var tickLen = convertTickWidth(gd, xa, trace);
     var minDiff = trace._minDiff;
-    delete trace._minDiff;
+    trace._minDiff = null;
     var x = trace._xcalc;
-    delete trace._xcalc;
+    trace._xcalc = null;
 
     var cd = calcCommon(gd, trace, x, ya, ptFunc);