Vega visualization Issue

Hi, I'm a new user using vega to customize my own dashboard. However, I'm facing some strange issues now...

what I'm trying to do is use vega to create a bar chart showing some results from the Elasticsearch data pool.

here is my vega script:

and there is nothing shown on the right side of the visualization window.
I have checked the inspector window, it shows that there has response.

Can someone tell me what I did wrong? I simply have no idea how to fix this...

And here are screenshots of the inspector result: