" diagrammatic and graphic presentation serve only preliminary conclusions" what does this means..

The diagrammatic and graphic presentations serve only preliminary conclusions in the sense that they only provide an overview of the real solutions. To put in different words, with graphic presentations, only the broad conclusions can be drawn rather than the exact ones. For instance, by looking at a pie chart you can just estimate the share of each section, however, their exact values can be drawn only by calculating the percentage shares.