That's a misleading chart. What it means is we are seeing more warm records in relation to cold records. It doesn't say anything about the total number of warm records. North America has been seeing milder winters these past thirty years. It doesn't mean we've been seeing more record highs than other periods.

They explain this here: "By extension, there has been an increase in the ratio of the number of record highs to record lows (Figure 6.5). Over the past two decades, the average of this ratio exceeds two (meaning that twice as many high-temperature records have been set as low-temperature records)."