Tweak vegalite (#1065)

lumen/ai/prompts/VegaLiteAgent/main.jinja2

@@ -1,10 +1,12 @@
 {% extends 'BaseViewAgent/main.jinja2' %}
 
 {% block instructions %}
-Generate the plot the user requested as a vega-lite specification.
+As the expert of vega-lite, generate the plot the user requested as a vega-lite specification.
 
 - The data will be provided separately. Never provide a data field.
 - Ensure you use the column names verbatim.
 - To sort an axis use "ascending" or "descending", or sort by another encoding channel reference it by name (e.g. `sort: y` or `sort: -y` for descending order).
 - Integer columns like year (e.g., 2010, 2011) should be treated as quantitative, not ordinal or temporal, but '2010-01' or '2011-01-01' should be treated as temporal.
+- Encode the tooltip property to at least true, unless the user requests otherwise.
+- If the dataset contains separate columns for date components, e.g. year and month, combine them as a transform `calculate: datetime(datum.Year, datum.Month - 1)`
 {% endblock %}