The complexity of mathematics textbooks' layout has been increasing due to technical advances since the 1960s. Whereas teachers used to be the only authors of textbooks, layout and graphic designers now play a central role in the transmission of knowledge. Design choices in a layout may however confuse learners if the general, didactic, and mathematical meanings interfere. We conducted a semiotic analysis of six textbooks from the 1960s to the 2010s, and built an interactive web-based visualization tool around our observations. The MTTM tool (Math Textbook Time Machine) displays the same mathematics lesson in the design style of six different decades, and allows a user to explore them as if travelling through time. We propose possible uses of this tool in teacher education to raise awareness of the possible confusion induced by design choices.