diff --git a/gitimerge.py b/gitimerge.py
index d55839f..e0853ad 100644
--- a/gitimerge.py
+++ b/gitimerge.py
@@ -1456,6 +1456,67 @@ def find_frontier_blocks(block):
                 )
 
 
+def write_diagram_with_axes(f, diagram, tip1, tip2):
+    """Write a diagram of one-space-wide characters to file-like object f.
+
+    Include integers along the top and left sides showing the indexes
+    corresponding to the rows and columns.
+
+    """
+
+    len1 = len(diagram)
+    len2 = len(diagram[0])
+
+    # Write the line of i1 numbers:
+    f.write('   ')
+    for i1 in range(0, len1, 5):
+        f.write('%5d' % (i1,))
+
+    if (len1 - 1) % 5 == 0:
+        # The last multiple-of-five integer that we just wrote was
+        # the index of the last column. We're done.
+        f.write('\n')
+    else:
+        if (len1 - 1) % 5 == 1:
+            # Add an extra space so that the numbers don't run together:
+            f.write(' ')
+        f.write('%s%d\n' % (' ' * ((len1 - 1) % 5 - 1), len1 - 1,))
+
+    # Write a line of '|' marks under the numbers emitted above:
+    f.write('   ')
+    for i1 in range(0, len1, 5):
+        f.write('%5s' % ('|',))
+
+    if (len1 - 1) % 5 == 0:
+        # The last multiple-of-five integer was at the last
+        # column. We're done.
+        f.write('\n')
+    elif (len1 - 1) % 5 == 1:
+        # Tilt the tick mark to account for the extra space:
+        f.write(' /\n')
+    else:
+        f.write('%s|\n' % (' ' * ((len1 - 1) % 5 - 1),))
+
+    # Write the actual body of the diagram:
+    for i2 in range(len2):
+        if i2 % 5 == 0 or i2 == len2 - 1:
+            f.write('%4d - ' % (i2,))
+        else:
+            f.write('       ')
+
+        for i1 in range(len1):
+            f.write(diagram[i1][i2])
+
+        if tip1 and i2 == 0:
+            f.write(' - %s\n' % (tip1,))
+        else:
+            f.write('\n')
+
+    if tip2:
+        f.write('       |\n')
+        f.write('     %s\n' % (tip2,))
+
+
 class MergeFrontier(object):
     """Represents the merge frontier within a Block.
 
@@ -1696,13 +1757,10 @@ def format_diagram(self, formatter=None, diagram=None):
             [formatter(diagram[i1][i2]) for i2 in range(self.block.len2)]
             for i1 in range(self.block.len1)]
 
-    def write(self, f):
+    def write(self, f, tip1=None, tip2=None):
         """Write this frontier to file-like object f."""
-        diagram = self.format_diagram()
-        for i2 in range(self.block.len2):
-            for i1 in range(self.block.len1):
-                f.write(diagram[i1][i2])
-            f.write('\n')
+
+        write_diagram_with_axes(f, self.format_diagram(), tip1, tip2)
 
     def write_html(self, f, name, cssfile='imerge.css', abbrev_sha1=7):
         class_map = {
@@ -2279,16 +2337,17 @@ def format_diagram(self, legend=None, diagram=None):
             [legend[diagram[i1][i2]] for i2 in range(self.len2)]
             for i1 in range(self.len1)]
 
-    def write(self, f, legend=None, sep='', linesep='\n'):
-        diagram = self.format_diagram(legend)
-        for i2 in range(self.len2):
-            f.write(sep.join(diagram[i1][i2] for i1 in range(self.len1)) + linesep)
+    def write(self, f, tip1='', tip2=''):
+        write_diagram_with_axes(f, self.format_diagram(), tip1, tip2)
 
     def writeppm(self, f):
+        legend = ['127 127 0', '0 255 0', '0 127 0', '255 0 0', '127 0 0']
+        diagram = self.format_diagram(legend)
+
         f.write('P3\n')
         f.write('%d %d 255\n' % (self.len1, self.len2,))
-        legend = ['127 127 0', '0 255 0', '0 127 0', '255 0 0', '127 0 0']
-        self.write(f, legend, sep='  ')
+        for i2 in range(self.len2):
+            f.write('  '.join(diagram[i1][i2] for i1 in range(self.len1)) + '\n')
 
 
 class SubBlock(Block):
@@ -3702,11 +3761,11 @@ def cmd_diagram(parser, options):
 
     merge_state = read_merge_state(git, options.name)
     if options.commits:
-        merge_state.write(sys.stdout)
+        merge_state.write(sys.stdout, merge_state.tip1, merge_state.tip2)
         sys.stdout.write('\n')
     if options.frontier:
         merge_frontier = MergeFrontier.map_known_frontier(merge_state)
-        merge_frontier.write(sys.stdout)
+        merge_frontier.write(sys.stdout, merge_state.tip1, merge_state.tip2)
         sys.stdout.write('\n')
     if options.html:
         merge_frontier = MergeFrontier.map_known_frontier(merge_state)