String literals are an array of const char, we can see this from the draft C++ standard section 2.14.5
String literals which says (emphasis mine):
Ordinary string literals and UTF-8 string literals are also referred to as narrow string literals. A narrow string literal has type “array of n const char”, where n is the size of the string as defined below, and has static storage duration (3.7).
so this change will remove the warning:
const char * commandsForGnuplot[] = {"set title \"Probability Graph\"", "plot 'data.temp' with lines"};
^^^^^
Note, allowing a *non-const char** to point to const data is a bad idea since modifying a const or a string literal is undefined behavior. We can see this by going to section 7.1.6.1
The cv-qualifiers which says:
Except that any class member declared mutable (7.1.1) can be modified,
any attempt to modify a const object during its lifetime (3.8) results
in undefined behavior.
and section 2.14.5
String literals which says:
Whether all string literals are distinct (that is, are stored in
nonoverlapping objects) is implementation defined. The effect of
attempting to modify a string literal is undefined.