Clean up plot tic close to zero

On 8/8/2024 on this commit:
8adaf0e
I took out logic that formatted tics
close to zero as zero and instead used
the actual tic value no matter how
close to zero.

This introduced another issue
when tic values were close to zero
but actually meant to be zero.

For example if Tics={-1e-17,1e-34,1e-17},
the plot should really have
Tics={-1e-17,0,1e-17}.

This commit fixes that issue by replacing
the tic closest to zero with zero if
appropriate.

libkoviz/bookmodel.cpp

@@ -3231,6 +3231,31 @@ QList<double> PlotBookModel::_calcTicSet(double aIn, double bIn,
         }
     }
 
+    // Clean up X by replacing single tic that is nearest zero with zero
+    // For example, if X={-1e-17,1e-34,1e-17}, make X={-1e-17,0,1e-17}
+    // Only replace tic if:
+    //    1. a <= 0 <= b
+    //    2. |X| > 1
+    //    3. log10(tic) is less than another tic
+    if ( a <= 0 && b >= 0 && X.size() > 1 ) {
+        int idx_nearest_zero = 0;
+        double xmin = DBL_MAX;
+        for ( int i = 0; i < X.size(); ++i ) {
+            double x = X.at(i);
+            if ( qAbs(x) < xmin ) {
+                xmin = qAbs(x);
+                idx_nearest_zero = i;
+            }
+        }
+        if ( xmin != 0 ) {
+            int i = ( idx_nearest_zero == 0 ) ? 1 : 0;
+            double other_tic = X.at(i);
+            if ( log10(qAbs(xmin)) < log10(qAbs(other_tic))-1 ) {
+                X.replace(idx_nearest_zero,0.0);
+            }
+        }
+    }
+
     // Shrink tic set until |X| <=7
     // Tics are removed in favor of keeping tics "even" in order
     // to keep them the same while panning