[QC-347] Error bars for graphs drawn by TrendingTask (#453)

* [QC-347] Error bars for graphs drawn by TrendingTask

* more comments, fixing the existing ones

* add documentation

Author: knopers8

Framework/include/QualityControl/TrendingTaskConfig.h

@@ -36,6 +36,7 @@ struct TrendingTaskConfig : PostProcessingConfig {
     std::string varexp;
     std::string selection;
     std::string option;
+    std::string graphErrors;
   };
 
   struct DataSource {

Framework/src/TrendingTask.cxx

@@ -23,6 +23,7 @@
 #include <TH1.h>
 #include <TCanvas.h>
 #include <TPaveText.h>
+#include "TGraphErrors.h"
 
 using namespace o2::quality_control;
 using namespace o2::quality_control::core;
@@ -113,15 +114,34 @@ void TrendingTask::storePlots()
   // why generate and store plots in the same function? because it is easier to handle the lifetime of pointers to the ROOT objects
   for (const auto& plot : mConfig.plots) {
 
+    // we determine the order of the plot, i.e. if it is a histogram (1), graph (2), or any higher dimension.
+    const size_t plotOrder = std::count(plot.varexp.begin(), plot.varexp.end(), ':') + 1;
+    // we have to delete the graph errors after the plot is saved, unfortunately the canvas does not take its ownership
+    TGraphErrors* graphErrors = nullptr;
+
     TCanvas* c = new TCanvas();
 
     mTrend->Draw(plot.varexp.c_str(), plot.selection.c_str(), plot.option.c_str());
 
     c->SetName(plot.name.c_str());
     c->SetTitle(plot.title.c_str());
 
+    // For graphs we allow to draw errors if they are specified.
+    if (!plot.graphErrors.empty()) {
+      if (plotOrder != 2) {
+        ILOG(Error) << "Non empty graphErrors seen for the plot '" << plot.name << "', which is not a graph, ignoring." << ENDM;
+      } else {
+        // We generate some 4-D points, where 2 dimensions represent graph points and 2 others are the error bars
+        std::string varexpWithErrors(plot.varexp + ":" + plot.graphErrors);
+        mTrend->Draw(varexpWithErrors.c_str(), plot.selection.c_str(), "goff");
+        graphErrors = new TGraphErrors(mTrend->GetSelectedRows(), mTrend->GetVal(1), mTrend->GetVal(0), mTrend->GetVal(2), mTrend->GetVal(3));
+        // We draw on the same plot as the main graph, but only error bars
+        graphErrors->Draw("SAME E");
+      }
+    }
+
     // Postprocessing the plot - adding specified titles, configuring time-based plots, flushing buffers.
-    // Notice that axes and title is drawn using a histogram, even in the case of graphs.
+    // Notice that axes and title are drawn using a histogram, even in the case of graphs.
     if (auto histo = dynamic_cast<TH1*>(c->GetPrimitive("htemp"))) {
       // The title of histogram is printed, not the title of canvas => we set it as well.
       histo->SetTitle(plot.title.c_str());
@@ -134,7 +154,7 @@ void TrendingTask::storePlots()
         // It will have an effect only after invoking Draw again.
         title->Draw();
       } else {
-        ILOG(Info) << "Could not get the title TPaveText of the plot '" << plot.name << "'." << ENDM;
+        ILOG(Error) << "Could not get the title TPaveText of the plot '" << plot.name << "'." << ENDM;
       }
 
       // We have to explicitly configure showing time on x axis.
@@ -151,7 +171,7 @@ void TrendingTask::storePlots()
       // so we have to do it here.
       histo->BufferEmpty();
     } else {
-      ILOG(Info) << "Could not get the htemp histogram of the plot '" << plot.name << "'." << ENDM;
+      ILOG(Error) << "Could not get the htemp histogram of the plot '" << plot.name << "'." << ENDM;
     }
 
     auto mo = std::make_shared<MonitorObject>(c, mConfig.taskName, mConfig.detectorName);
@@ -160,5 +180,7 @@ void TrendingTask::storePlots()
 
     // It should delete everything inside. Confirmed by trying to delete histo after and getting a segfault.
     delete c;
+    // ...but it does not delete graphErrors
+    delete graphErrors;
   }
 }

Framework/src/TrendingTaskConfig.cxx

@@ -27,7 +27,8 @@ TrendingTaskConfig::TrendingTaskConfig(std::string name, const boost::property_t
                       plotConfig.second.get<std::string>("title", ""),
                       plotConfig.second.get<std::string>("varexp"),
                       plotConfig.second.get<std::string>("selection", ""),
-                      plotConfig.second.get<std::string>("option", "") });
+                      plotConfig.second.get<std::string>("option", ""),
+                      plotConfig.second.get<std::string>("graphErrors", "") });
   }
   for (const auto& dataSourceConfig : config.get_child("qc.postprocessing." + name + ".dataSources")) {
     if (const auto& sourceNames = dataSourceConfig.second.get_child_optional("names"); sourceNames.has_value()) {
@@ -51,4 +52,4 @@ TrendingTaskConfig::TrendingTaskConfig(std::string name, const boost::property_t
   }
 }
 
-} // namespace o2::quality_control::postprocessing
+} // namespace o2::quality_control::postprocessing

doc/PostProcessing.md

@@ -192,8 +192,10 @@ Data sources are defined by filling the corresponding structure, as in the examp
 }
 ```
 
-Similarly, plots are defined by adding proper structures to the `"plots"` list, as shown below. The plot will be stored under the `"name"` value and it will have the `"title"` value shown on the top. The `"varexp"`, `"selection"` and `"option"` fields correspond to the arguments of the [`TTree::Draw`](https://root.cern/doc/master/classTTree.html#a73450649dc6e54b5b94516c468523e45) method.
-
+Similarly, plots are defined by adding proper structures to the `"plots"` list, as shown below. The plot will be
+ stored under the `"name"` value and it will have the `"title"` value shown on the top. The `"varexp"`, `"selection"` and `"option"` fields correspond to the arguments of the [`TTree::Draw`](https://root.cern/doc/master/classTTree.html#a73450649dc6e54b5b94516c468523e45) method.
+Optionally, one can use `"graphError"` to add x and y error bars to a graph, as in the first plot example.
+The `"name"` and `"varexp"` are the only compulsory arguments, others can be omitted to reduce configuration files size.
 ``` json
 {
         ...
@@ -203,7 +205,8 @@ Similarly, plots are defined by adding proper structures to the `"plots"` list,
             "title": "Mean trend of the example histogram",
             "varexp": "example.mean:time",
             "selection": "",
-            "option": "*L"
+            "option": "*L",
+            "graphErrors": "5:example.stddev"
           },
           {
             "name": "histogram_of_means",