加载外部资源

• d3.xhr - request a resource using XMLHttpRequest.
• xhr.header - set a request header.
• xhr.mimeType - set the Accept request header and override the response MIME type.
• xhr.response - set a response mapping function.
• xhr.get - issue a GET request.
• xhr.post - issue a POST request.
• xhr.send - issue a request with the specified method and data.
• xhr.abort - abort an outstanding request.
• xhr.on - add an event listener for "progress", "load" or "error" events.
• d3.text - request a text file.
• d3.json - request a JSON blob.
• d3.html - request an HTML document fragment.
• d3.xml - request an XML document fragment.
• d3.csv - request a comma-separated values (CSV) file.
• d3.tsv - request a tab-separated values (TSV) file.