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diff --git a/.gitignore b/.gitignore
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+*.csv
diff --git a/README.md b/README.md
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+# plotasn.py
+
+This is a simple python application to plot the balance of your [ASN][] bank
+accounts. It combines lines of the individual accounts and their total sum in a
+single graph.
+
+Download CSV exports from the online environment and run:
+
+```bash
+./plotasn.py account-1.csv [account-2.csv ...]
+```
+
+This is tool is hereby placed in the public domain.
+
+[ASN]: https://www.asnbank.nl/
diff --git a/plotasn.py b/plotasn.py
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+#!/usr/bin/env python3
+
+from argparse import ArgumentParser
+import csv
+import datetime as dt
+import matplotlib.pyplot as plt
+import matplotlib.dates as mdates
+import numpy as np
+
+class Record:
+    def __init__(self, date, balance):
+        if type(date) == str:
+            self.datetime = dt.datetime.strptime(date, '%d-%m-%Y')
+        else:
+            self.datetime = date
+        self.balance = balance
+
+    def __repr__(self):
+        return str(self.datetime) + ': ' + str(self.balance)
+
+def read_csv(f):
+    """Read a CSV file into a list of records."""
+    records = []
+    with open(f) as csvfile:
+        rdr = csv.reader(csvfile)
+        for row in rdr:
+            before = float(row[8])
+            after = before + float(row[10])
+            records.append(Record(row[11], before))
+            records.append(Record(row[11], after))
+    return records
+
+def remove_day_internal_records(records):
+    """Keeps only the first and last record for each day."""
+    i = 0
+    while i < len(records):
+        if i + 2 >= len(records):
+            break
+        elif records[i].datetime == records[i+2].datetime:
+            del records[i+1]
+        else:
+            records[i+1].datetime = records[i+1].datetime.replace(hour=23, minute=59, second=59)
+            i += 2
+    if i + 1 < len(records):
+        records[i+1].datetime = records[i+1].datetime.replace(hour=23, minute=59, second=59)
+
+def add_limits(record_lists):
+    """Add records for the start and end of all record lists.
+
+    This fixes files that do not cover the same interval.
+    """
+    mindate = record_lists[0][0].datetime
+    maxdate = record_lists[0][-1].datetime
+
+    for rs in record_lists:
+        mindate = min(mindate, rs[0].datetime)
+        maxdate = max(maxdate, rs[-1].datetime)
+
+    for rs in record_lists:
+        # We add two because this still goes through remove_day_internal_records
+        rs.insert(0, Record(mindate, rs[0].balance))
+        rs.insert(0, Record(mindate, rs[0].balance))
+        rs.append(Record(maxdate, rs[-1].balance))
+        rs.append(Record(maxdate, rs[-1].balance))
+
+def sum_records(axes_lists):
+    """Return a new axes pair for the sum of all given axes pairs."""
+    for i in range(len(axes_lists)):
+        axes_lists[i][0] = [x.timestamp() for x in axes_lists[i][0]]
+    xs = np.unique(np.concatenate([xy[0] for xy in axes_lists]))
+    yss = [np.interp(xs, xy[0], xy[1], left=0, right=0) for xy in axes_lists]
+    ys = sum(yss)
+    return [dt.datetime.fromtimestamp(x) for x in xs], ys
+
+def records_to_axes(records):
+    """Convert a list of records to an axes pair."""
+    xs = []
+    ys = []
+    for r in records:
+        xs.append(r.datetime)
+        ys.append(r.balance)
+    return xs, ys
+
+def plot_files(files):
+    record_lists = [read_csv(f) for f in files]
+    add_limits(record_lists)
+
+    axes_lists = []
+    for rs in record_lists:
+        remove_day_internal_records(rs)
+        xs, ys = records_to_axes(rs)
+        axes_lists.append([xs,ys])
+
+    i = 0
+    for xs, ys in axes_lists:
+        plt.plot(xs, ys, label=files[i])
+        i += 1
+
+    if len(files) > 1:
+        xs, ys = sum_records(axes_lists)
+        plt.plot(xs, ys, label='Sum')
+
+    plt.legend()
+    plt.gca().xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
+    plt.gca().set_ylim((0, None))
+    plt.gcf().autofmt_xdate()
+    plt.show()
+
+def main():
+    p = ArgumentParser(description='Plot balances of ASN bank accounts')
+    p.add_argument('files', metavar='FILE', type=str, nargs='+',
+            help='a CSV export of an ASN bank account')
+    args = p.parse_args()
+
+    plot_files(args.files)
+
+if __name__ == '__main__':
+    main()