Converting click count data to JSON

I have an array filled with 5 other arrays that consists of arrays of 2 values (first a date then a count):

[ 
  [ 
    [dt, cnt], [dt, cnt], .... 
  ], 
  [ 
    [dt, cnt], [dt, cnt], .... 
  ], 
  [ 
    [dt, cnt], [dt, cnt], .... 
  ], 
  [ 
    [dt, cnt], [dt, cnt], .... 
  ], 
  [ 
    [dt, cnt], [dt, cnt], .... 
  ], 
]

It concerns click statistics of different websites. This data needs to be converted to data to make a consistent chart with Google visualisations. So first a conversion is done in Python, then this result converted to JSON to pass to the Google libs.

My current code is this:

# determine min date
mindate = datetime.date.max
for dataSet in sets:
    if (dataSet[0][0] < mindate):
        mindate = dataSet[0][0];
# fill a dictionary with all dates involved
datedict = {}
for dat in daterange(mindate, today):
    datedict[dat] = [dat];
# fill dictionary with rest of data
arrlen = 2
for dataSet in sets:
    # first the values
    for value in dataSet:
        datedict[value[0]] = datedict[value[0]] + [value[1]];
    # don't forget the missing values (use 0)
    for dat in daterange(mindate, today):
        if len(datedict[dat]) < arrlen:
            datedict[dat] = datedict[dat] + [0]
    arrlen = arrlen + 1
# convert to an array
datearr = []
for dat in daterange(mindate, today):
    datearr = datearr + [datedict[dat]]
# convert to json
result = json.dumps(datearr, cls=DateTimeEncoder)

(The DateTimeEncoder just gives a JavaScript-friendly datetime in the JSON)

The output looks like this:

[
    ["new Date(2008, 7, 27)", 0, 5371, 1042, 69, 0], 
    ["new Date(2008, 7, 28)", 0, 5665, 1100, 89, 0], 
    ...
]

This is one of my first Python adventures, so I expect this piece of code can be improved upon easily. I want this to be shorter and more elegant. Show me the awesomeness of Python because I'm still a bit disappointed.

I'm using Django, by the way.

Trailing semicolons

At many places there is a ; at the end of the line.
It's completely unnecessary in Python and you should remove those.

Appending to arrays

At several places you are appending values to arrays in a strange way, for example:

datedict[val[0]] = datedict[val[0]] + [val[1]];

The common and shorter way to do this is using .append():

datedict[val[0]].append(val[1])

Augmented assignment operator +=

Instead of:

l = l + 1

This is shorter and better:

l += 1

Use list comprehensions

Instead of:

datearr = []
for dat in daterange(mindate, today):
    datearr = datearr + [datedict[dat]]

You can use a list comprehension, a powerful feature of Python:

datearr = [datedict[dat] for dat in daterange(mindate, today)]

StackExchange Code Review Q#78248, answer score: 4