When changing from Portrait to Landscape view, graphs sometimes become truncated. To adjust your view: Tap your screen to bring up the navigation bar. Select Text Settings (you may need to click the three dots for "more" on the navigation bar for this to show up). Within the Text Settings you can adjust the Font size.
*This change may only be done to Epub's and not PDF's. The reason for this is, a PDF is a snapshot of a page in a physical book or journal. Charts, images, and diagrams will appear the same way they appear in the printed book. This is great for picture books as well as others where the design is key. If you enlarge the page, you are zooming in to a section; this sometimes makes it not a practical option for people with low vision.
With EPUB, the emphasis is on the text itself. You can resize the text in either direction, to make short pages with large text or long pages with tiny text.