Visualizing the Most Common Unisex Names in the Us

Actually the grammatical distinction between masculine and feminine nouns had appeared earlier in Afro-Asiatic (including Semitic) languages, and only later in Indo-European languages (perhaps caused by the contact with Semitic languages), where previously a grammatical distinction existed only between the names of animate things and non-animate things. By the time of Ancient Greek and Latin, the grammatical distinction between masculine and feminine nouns was already well entrenched in the European languages.

Many centuries later, a part of the European languages, including English, have lost the word terminations that distinguished the masculine names from the feminine names. Only then formerly different masculine and feminine names have merged into a single unisex name.

So there is no surprise in your observation, as it is caused by the difference in behavior between names that have been fixed in writing at an earlier time, preserving an older pronunciation, which included specifically feminine word terminations, and names that have been fixed in writing more recently, when there no longer existed different masculine and feminine name declensions.

I think a traditional vertical stacked bar chart would have been far better.