When we are drawing images on the fly we have to tell the browser about the content we are sending to it. We can't display an image by telling the browser that we are sending text or html code. To this we have to use header declaration at the first line of data we send to the browser window.
Without this line at the starting of the data (not the starting of the page) if we send and image browser will display non recognized characters on the screen. So we have to tell browser that we are sending an image. We also can't send any text or data along with the image. So we will be using another window to display any graph or image not the existing window were some text or data is already there. We also can't mix both in the new window.