Automate png formatting with python

Pillow (the maintained successor to PIL: Python Imaging Library) can handle exactly what you want.

You can extend your image and place the text after retrieving user input. Here is an example for adding the title:

from PIL import Image, ImageFont, ImageDraw

img = Image.open('my_chart.png')
w,h= img.size

# put pixels into 2D array for ease of use
data = list(img.getdata())
xy_data = []
for y in xrange(h):
    temp = []
    for x in xrange(w):
        temp.append(data[y*w + x])
    xy_data.append(temp)

# get the title
title = raw_input("Title:")

# load the font
font_size = 20
font = ImageFont.truetype("/path/to/font.ttf",font_size)

#  Get the required height for you images
height_needed = font.getsize(title)[1] + 2  # 2 px for padding

# get the upperleft pixel to match color
bg = xy_data[0][0]

# add rows to the data to prepare for the text
xy_data = [[bg]*w for i in range(height_needed+5)] + xy_data  # +5 for more padding

# resize image
img = img.resize((w,h+height_needed+5))

# convert data back to 1D array
data = []
for line in xy_data:
    data += line

# put the image back in the data
img.putdata(data)

# get the ImageDraw item for this image
draw = ImageDraw.Draw(img)

# draw the text
draw.text((5,0),title,font=font,fill=(0,0,0))  # fill is black

img.save('titled_plot.png')