[metrics-bugs] #25264 [Metrics/Website]: Decide what graph to display when there's no data to graph

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#25264: Decide what graph to display when there's no data to graph
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 Reporter:  karsten          |          Owner:  metrics-team
     Type:  enhancement      |         Status:  new
 Priority:  Low              |      Milestone:
Component:  Metrics/Website  |        Version:
 Severity:  Normal           |     Resolution:
 Keywords:                   |  Actual Points:
Parent ID:                   |         Points:
 Reviewer:                   |        Sponsor:
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Comment (by iwakeh):

 Replying to [comment:3 karsten]:
 > Replying to [comment:2 iwakeh]:
 > > For avoiding the 500 server error when trying to access the png (as
 reported in detail in #25468) an empty graph/png stating 'no data
 available for this parameter choice' should be generated.
 >
 > Sounds good. We'll also need such a static file in the PDF format. I'll
 create placeholders using R/ggplot2 in a bit. We just need to write the
 code to put them in.
 >
 > > Currently, the corresponding csv only contains the header.  Maybe,
 also add a comment here too in case no data is available.
 >
 > Sounds good, too. Want to suggest a text? I can put it in then.
 >
 > > It should be avoided to call R if there is no data available.
 >
 > Well, we need to call R to find out whether there's data available for
 the requested period of time. Or what did you mean?

 The java code currently returns the `SC_INTERNAL_SERVER_ERROR`.  This is
 decided by the properties of the RObject received.  As RObject is our code
 it should always be available in order to avoid the null check.
 In case, the R code doesn't give a result or fails the RObject should
 'know', i.e., a method needs to be added, e.g., `boolean error()` for any
 unforeseeable error w/o any file returned (triggers error message)
 otherwise (in case of error()==false) it returns the wanted file or the
 'no-data' version.

 Regarding the text:

    Unfortunately there is no data available for the chosen parameters.

 Is fine for all and we should include the parameters chosen into the
 response, e.g., in the pdf, csv, png.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25264#comment:5>
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