Fix display of variable lines with no variation or with invalid values

Hello,

If a variable line contains only one value or if it contains many values, but they are all the same, there is a bug in displaying the line. There is also the same graphical bug if one of line values is nan or inf:

This PR fixes these two buggy cases:

src/common/Tools.cpp

@@ -57,6 +57,11 @@
 using namespace std;
 
 bool convert_to_double(const std::string &arg, double *val) {
+    if (arg.compare("-nan") == 0 || arg.compare("nan") == 0 || arg.compare("inf") == 0) {
+        *val = 0;
+        return true;
+    }
+
     int nb_read;
     // Try to convert first in the current locale
     sscanf(arg.c_str(), "%lf%n", val, &nb_read);