Method has_alpha

source/matplot/axes_objects/matrix.cpp:337–339

    bool matrix::is_rgba() const { return matrices_.size() == 4; }

    bool matrix::has_alpha() const {
        return matrices_.size() == 4 || alpha_ != 0.;
    }

    const std::vector<std::vector<double>> &matrix::matrix_r() const {
        return matrices_[0];