fix.cpp

// construct keyword args
PyObject* kwargs = PyDict_New();
for(std::map<std::string, std::string>::const_iterator it = keywords.begin(); it != keywords.end(); ++it) {
    // recommendation from: https://github.com/lava/matplotlib-cpp/issues/75
    if (it->first == "alpha")
        PyDict_SetItemString(kwargs, it->first.c_str(), PyFloat_FromDouble(std::stod(it->second)));
    else 
        PyDict_SetItemString(kwargs, it->first.c_str(), PyUnicode_FromString(it->second.c_str()));
}