Not able to display the Histogram properly from a function in c++ - c++

Question

I want to write a function to display a Histogram in C++ Visual Studio, The code is displaying a function to calculate histogram and display it, but I'm getting the same histogram for all the images used. The code is written below, Kindly advise where i might need improvement. Thank You.
class Histogram {
public:
Mat calc_histogram(Mat src) {
Mat hist;
hist = Mat::zeros(256, 1, CV_32F);
src.convertTo(src, CV_32F);
double value{ 0 };
for (int i = 0; i < src.rows; i++)
{
for (int j = 0; j < src.cols; j++)
{
value = src.at<float>(i, j);
//Add 1 point to the bin pixel intensity, so giving total pixels at each intensities
hist.at<float>(value) = hist.at<float>(value) + 1;
}
}
return hist;
}
void plot_histogram(Mat histogram) {
Mat histogram_image(400, 512, CV_8UC3, Scalar(0, 0, 0)); //Image background to draw hist on &
//Scaling it 512 pixels for clearer image
Mat normalized_histogram;
normalize(histogram, normalized_histogram, 0, 400, NORM_MINMAX, -1, Mat());
for (int i{ 0 }; i < 256; i++)
{
rectangle(histogram_image, Point(2 * i, histogram_image.rows - normalized_histogram.at<float>(i)),
Point(2 * (i + 1), histogram_image.rows), Scalar(255, 0, 0));
}
//namedWindow("Histogtram", WINDOW_NORMAL);
imshow("Histogram", histogram_image);
}
};
int main() {
Mat img;
img = imread("Shiv_Mahadev.png");
Histogram H1;
Mat hist = H1.calc_histogram(img);
H1.plot_histogram(hist);
waitKey(0);
destroyAllWindows();
return 0;
}