How to make a histogram

So i want to make a histogram from the pixelvalues of an image and it just takes forever.

my histogram function is the following:

def histogram(pixelcount):
    '''
    displays the histogram of the current image
    '''

    fig=go.Figure()
    fig.add_trace(go.Histogram(x=pixelcount,
                               xbins=dict(size=5)))

    return fig

The input is a list created this way:

    imgy=Image.open(img, 'r')
    pix_val=list(imgy.getdata())
#i dont care about 0 values
    pix_count=[x for sets in pix_val for x in sets if x>0]

and creating the histogram takes several minutes.
I do not understand why. Am i passing the data wrong, or is there a faster way to make a histogram?
Sure the list is approx. 1.8 mio elements long, but the whole operation

    imgy=Image.open(img, 'r')
    pix_val=list(imgy.getdata())
    pix_count=[x for sets in pix_val for x in sets if x>0]
    for i in pix_val_flat:
        if i in pixel_dict.keys():
            pixel_dict[i]+=1
        else:
         pixel_dict.update({i:1})   

takes approximately a second, while creating the histogram takes several minutes.

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