  "mempoolminfee": 0.00000000

When the minrelaytxfee is > 0.

{
    if (request.fHelp || request.params.size() > 2)
        throw std::runtime_error(
            "getchaintxstats ( nblocks blockhash )\n"
            "\nCompute statistics about the total number and rate of transactions in the chain.\n"
            "\nArguments:\n"
            "1. nblocks      (numeric, optional) Size of the window in number of blocks (default: one month).\n"
            "2. \"blockhash\"  (string, optional) The hash of the block that ends the window.\n"
            "\nResult:\n"
            "{\n"