I pretty much agree with this. They created a D3.js treemap and probably even used a standard Colorbrewer blue color scheme, as seen in this d3 example:
Was it terribly uncreative for the other YCombinator company to do the same thing? Yes.
Was it plagiarism, copying, or unethical? No way. You did not invent the treemap nor a strong blue on light blue color scheme. Your UX is not unique enough to claim design infringement here.
Lastly, your TOS is probably the most alarming part of this story. You should not be so afraid of competition that you attempt to prohibit 'competitors' from using your product.
http://bl.ocks.org/mbostock/4063582
Was it terribly uncreative for the other YCombinator company to do the same thing? Yes.
Was it plagiarism, copying, or unethical? No way. You did not invent the treemap nor a strong blue on light blue color scheme. Your UX is not unique enough to claim design infringement here.
Lastly, your TOS is probably the most alarming part of this story. You should not be so afraid of competition that you attempt to prohibit 'competitors' from using your product.