When comparing Events in a Formulas* report, the x-axis will show decimals and not percentages. This is because not all Formulas produce percentages (depending on what Events you’re comparing, some will produce absolutes).
However, if your Formula does yield a decimal and you’d like it to be a percentage in the chart instead, you can multiply the numerator by 100:
*Limited by plan type
Access to the Formulas report is available for customers on a Startup or Enterprise plan - visit the pricing page for more information or to upgrade.