According to the National Centers for Environmental Information (NCEI), the average April temperature in Illinois has warmed by 2 degrees over the last century. In addition, the most recent decade of 2006-2015 was the warmest set of Aprils on record. Statewide records go back to 1895.
Here are the statewide average temperatures for April in Illinois plotted by year. The purple line and black dots show the year-to-year variability in April temperatures in Illinois. The blue line shows the upward trend. You can find the original data and graph here.
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