Get density of messages on distinct ID's

I've got a database with latitude and senderID. I want to plot latitudes in a histogram, which is easy enough, but the thing is that rather than just plot the number of occurences pr latitude, I want to plot the density of latitudes pr unique senderID:

Number of latitude rows/Number of unique userids at that latitude

So if I have the following latitudes and users:

|Latitude | User |

   70        1
   70        2
   70        3
   70        4
   70        5
   80        1
   80        1
   80        2

The density for latitude 70 is 1, while the density for latitude 80 is clearly 1.5.

If I would do this through a database query/pseudocode I would do something like:

SELECT count(latitudes) FROM messages WHERE latitude < lastbin+binwidth AND latitude > lastbin
SELECT count(distinct userid) FROM messages WHERE latitude < lastbin+binwidth AND latitude > lastbin

The density would then be count(latitudes)/count(distinct userid). The binwidth would be about 0.1.

I have some code that I'm not quite sure of:

con = lite.connect(databasepath)
binwidth = 1
latitudes = []
userids = []
densities = []
with con:
    cur = con.cursor()
    cur.execute('SELECT latitude, userid FROM Message ORDER BY latitude asc')
    con.commit()
    while True:
        tmp = cur.fetchone()
        if tmp != None:
            latitudes.append(float(tmp[0]))
            userids.append(tmp[1])
        else:
            break
    y = -1
    for x in range(0, len(latitudes)):
        y = y+1
        b = x-y
        if ((abs(latitudes[x])-abs(latitudes[b])) >= binwidth) or x == len(latitudes)-1:
            templatitudes = latitudes[x-y:x]
            tempuserids = userids[x-y:x]
            tempdensity = math.floor(len(templatitudes)/len(list(set(tempuserids))))
            tempdensities = [(templatitudes[0])]*int(tempdensity)
            densities.extend(tempdensities)
            y = -1
    plt.hist(densities, stacked=True, histtype='step', bins=np.arange((min(densities)), (max(densities)) + binwidth, binwidth))

The latitudes is in the range of -90 to 90. When I run this code I get a lack of values alongside +- 10, hence my uncertainty.

To get any help with this is probably a far stretch, but I just cant organize my thoughts to do this while I'm confident there are no errors. I'm sorry if I was unclear.